The Latin phrase novus actus interveniens – meaning “a new intervening act” – describes one of the most significant limiting principles in the law of tort and, to a lesser extent, criminal law. It refers to an event that occurs after the defendant’s wrongful act and is sufficiently independent to break the chain of causation between that act and the claimant’s damage. Where the doctrine applies, the defendant escapes liability for consequences that flow from the intervening event rather than from the original wrong.
Causation sits at the heart of any claim in tort. A claimant must demonstrate not only that the defendant owed a duty of care and breached it, but also that the breach caused the loss complained of. This guide focuses on what happens when the causal chain, once established, is allegedly severed by a subsequent event.
The doctrine performs an essential function. Without it, a defendant’s liability could stretch indefinitely into the future, capturing every misfortune that happened to follow the original wrong. As Lord Wright observed in The Oropesa [1943] P 32, the law must draw a line somewhere. Novus actus interveniens is one of the principal tools for drawing that line.
Historical development
Early foundations
The concept of an intervening act breaking the chain of causation has deep roots in the common law. English judges have long recognised that a defendant should not bear responsibility for harm that is truly attributable to an independent cause. However, the formal articulation of the doctrine as a distinct legal principle emerged gradually during the nineteenth and early twentieth centuries.
Early authorities tended to address the issue in practical rather than theoretical terms. Courts asked, in essence, whether the defendant’s act remained the “effective” or “dominant” cause of the harm, or whether some later event had supplanted it. The language of novus actus interveniens provided a convenient label for this inquiry, but the underlying question was always one of common sense and fairness.
The influence of foreseeability
The development of novus actus interveniens is closely intertwined with the broader evolution of foreseeability as a limiting principle in tort law. Following the landmark decision in Overseas Tankship (UK) Ltd v Morts Dock & Engineering Co Ltd (The Wagon Mound) [1961] AC 388, the Privy Council established that a defendant is liable only for damage of a kind that was reasonably foreseeable. This principle inevitably shaped the analysis of intervening acts: if the intervening event was itself foreseeable, it was far less likely to break the chain of causation.
Lord Reid’s speech in McKew v Holland and Hannen and Cubitts (Scotland) Ltd [1969] 3 All ER 1621 provided a further important development. He explained that if an injured person acts unreasonably in response to the original wrong, that unreasonable conduct may itself constitute a novus actus interveniens. The emphasis on reasonableness – judged by the standard of the ordinary person – linked the doctrine firmly to objective principles that pervade much of tort law.
The relationship between causation and novus actus interveniens
Factual and legal causation
To understand novus actus interveniens, one must first appreciate the two-stage analysis that English law applies to causation. The first stage is factual causation, typically assessed by the but-for test: would the harm have occurred but for the defendant’s breach? If the answer is no, factual causation is established.
The second stage is legal causation. Even where factual causation is satisfied, the law may decline to treat the defendant as responsible if the chain of events between breach and damage is too remote, too tenuous, or interrupted by an independent intervening cause. It is at this second stage that novus actus interveniens operates.
In Barnett v Chelsea and Kensington Hospital Management Committee [1969] 1 QB 428, for example, the hospital’s failure to examine a patient who presented with arsenic poisoning did not cause his death. The patient would have died regardless. The case failed at the factual causation stage, and the question of any intervening act did not arise. By contrast, where factual causation is established, a novus actus argument asks whether the causal link was subsequently broken by some new event.
The operative cause
Courts frequently frame the inquiry in terms of whether the defendant’s wrongful act remained the “operative cause” of the harm, or whether the intervening event became the sole effective cause. In Smith v Littlewoods Organisation Ltd [1987] AC 241, Lord Goff explained that the voluntary act of a third party is generally regarded as a novus actus interveniens. One should not ordinarily be held responsible in law for the deliberate acts of others.
However, the word “generally” carries significant weight. The law does not treat every intervening act as breaking the chain. The central question, as we shall see, is whether the intervention was sufficiently independent, voluntary, and unforeseeable to relieve the defendant of responsibility for the ultimate harm.
Categories of intervening act
The doctrine applies to three broad categories of intervening event: acts of the claimant, acts of third parties, and natural events. Each category raises distinct considerations, although the overarching principles of independence, voluntariness, and foreseeability apply across all three.
Acts of the claimant
An injured claimant who acts unreasonably in the aftermath of the defendant’s wrong may break the chain of causation through their own conduct. The leading authority is McKew v Holland and Hannen and Cubitts (Scotland) Ltd [1969] 3 All ER 1621. The claimant suffered a leg injury at work due to his employer’s negligence. His leg was left prone to giving way without warning. Despite this, he descended a steep staircase with no handrail and without assistance. His leg gave way, and he suffered a further serious injury.
The House of Lords held that the claimant’s decision to descend the staircase in those circumstances was unreasonable. It constituted a novus actus interveniens, and the employer was not liable for the additional injury. Lord Reid explained that a person is not entitled to disregard the effects of the original injury and expose themselves to a risk of further harm through their own unreasonable conduct.
However, an important qualification applies. Courts recognise that an injured person may not always act with perfect prudence. If the claimant’s response to the injury is within the range of reasonable behaviour – even if not the wisest possible course – the chain of causation remains intact. The law does not expect perfection. Indeed, in situations of emergency or distress, courts are particularly reluctant to characterise the claimant’s response as unreasonable. This principle connects with the broader concept of volenti non fit injuria.
The question of suicide and self-harm
A particularly sensitive area concerns cases where the claimant’s intervening act is self-harm or suicide. In Corr v IBC Vehicles Ltd [2008] UKHL 13, Mr Corr suffered a serious workplace accident that left him with severe depression. He subsequently took his own life. His widow brought a claim against the employer, arguing that the suicide was a consequence of the original injury.
The House of Lords held that the suicide did not break the chain of causation. The depression was a foreseeable consequence of the physical injury, and the suicide was in turn a foreseeable consequence of the depression. Crucially, Mr Corr’s mental illness meant that his decision to end his life could not be regarded as a truly voluntary and informed act. This decision illustrates the court’s willingness to take a nuanced approach to the question of voluntariness, particularly where mental health is engaged.
Acts of third parties
The voluntary, deliberate act of a third party is, in principle, the paradigm case of a novus actus interveniens. As Lord Goff observed in Smith v Littlewoods [1987] AC 241, a defendant will not ordinarily be liable for damage caused by the independent act of a stranger. This reflects the fundamental principle that individuals are responsible for their own conduct.
Nevertheless, the chain of causation is not broken where the third party’s intervention was reasonably foreseeable. In Stansbie v Troman [1948] 2 KB 48, a decorator left the front door of a house unlocked while he went out to buy materials. A thief entered and stole property. The Court of Appeal held that the decorator was liable. The very risk that his carelessness created – the risk of a break-in – had materialised. The thief’s intervention was entirely foreseeable and did not constitute a novus actus.
Similarly, in Dorset Yacht Co Ltd v Home Office [1970] AC 1004, Borstal officers negligently allowed young offenders to escape. The escapees damaged yachts moored nearby. The House of Lords held that the officers’ negligence was the effective cause of the damage. The boys’ conduct was the very thing that the officers’ supervision was meant to prevent. Consequently, it could not be treated as an independent, intervening act.
Negligent medical treatment as an intervening act
A particularly important sub-category concerns cases where the claimant receives negligent medical treatment following the defendant’s original wrong. The traditional rule – often traced to Hogan v Bentinck West Hartley Collieries (Owners) Ltd [1949] 1 All ER 588 – holds that subsequent medical negligence will only break the chain of causation if it is “so grossly negligent as to be a completely inappropriate response” to the original injury.
This is a high threshold. Ordinary medical negligence does not constitute a novus actus. In Webb v Barclays Bank Plc [2001] EWCA Civ 1141, the claimant suffered a knee injury at work and subsequently underwent a below-knee amputation that was performed negligently. The Court of Appeal held that the negligent surgery did not break the chain of causation. The original tortfeasor remained liable for the consequences, because the negligent treatment was not so extraordinary as to eclipse the original wrong.
However, this area was revisited in Jenkinson v Hertfordshire CC [2023] EWHC 872 (KB). Baker J permitted an amendment to the defence to raise a novus actus interveniens argument based on negligent surgical treatment. The judge questioned whether the “so grossly negligent” threshold accurately stated the law, suggesting that the test might be less demanding than previously assumed. Although the case concerned a procedural application rather than a full trial, it has prompted renewed academic and professional debate about the scope of the medical negligence exception.
Natural events and acts of God
The chain of causation may also be broken by an extraordinary natural event – sometimes described as an act of God. For a natural event to qualify as a novus actus interveniens, it must be unforeseeable and independent of the defendant’s wrong. A foreseeable storm or flood, for instance, will not break the chain if the defendant’s negligence created or increased the risk of harm from such an event.
In Carslogie Steamship Co Ltd v Royal Norwegian Government [1952] AC 292, a ship was damaged by the defendant’s negligence and subsequently sailed to the United States for repairs. While in port, the ship suffered further damage in a severe storm. The House of Lords held that the storm damage was entirely independent of the original collision and broke the chain of causation. The defendant was not liable for the storm damage.
This category of case tends to be relatively straightforward in principle, though the factual assessment of foreseeability can be challenging. The key question is always whether the natural event was an independent supervening cause, or whether the defendant’s negligence created the very conditions that made the harm from that event possible.
The test for novus actus interveniens
A composite inquiry
There is no single, mechanical test for determining whether an intervening act breaks the chain of causation. Instead, courts apply a composite inquiry that draws on several overlapping considerations. The principal factors, distilled from the case law, are as follows.
Foreseeability remains the most important single consideration. If the intervening act was reasonably foreseeable at the time of the defendant’s breach, it is unlikely to break the chain. Conversely, a wholly unforeseeable intervention is far more likely to qualify as a novus actus. As Lord Wright stated in The Oropesa, the question is whether the intervening act can fairly be regarded as a consequence of the original wrong, or whether it is “extraneous, something unwarrantable”.
Voluntariness is particularly relevant where the intervening act is that of a human agent – whether the claimant or a third party. A free, deliberate, and informed decision by an independent actor is the archetype of a novus actus. However, if the intervener acts under compulsion, duress, necessity, or diminished mental capacity, their act may not be treated as truly voluntary.
Independence asks whether the intervening act was genuinely separate from the consequences set in motion by the defendant’s wrong, or whether it was a natural and probable response to the situation that the defendant created. An act that flows naturally from the original wrong is unlikely to be treated as independent, even if performed by a third party.
Reasonableness features prominently where the claimant’s own conduct is in issue. As McKew illustrates, unreasonable behaviour by the claimant may constitute a novus actus. But the courts apply this test with a degree of latitude, recognising that injured persons do not always act with complete rationality.
The role of policy
Alongside these doctrinal factors, judicial reasoning in this area is inevitably influenced by broader considerations of fairness and policy. Courts must balance the principle that defendants should bear responsibility for the foreseeable consequences of their wrongs against the equally important principle that liability must have limits. The result is a fact-sensitive inquiry in which no single factor is determinative (lyonsdavidson.co.uk, 2024).
Indeed, Lord Bingham acknowledged in Environment Agency v Empress Car Co (Abertillery) Ltd [1999] 2 AC 22 that causation in law is ultimately a question of common sense, not a matter of scientific or philosophical analysis. This pragmatic approach permeates the courts’ treatment of novus actus interveniens.
Modern developments
Recent case law
The doctrine continues to generate significant litigation. Beyond the Jenkinson decision discussed above, recent years have seen courts grapple with novus actus arguments in a range of contexts — from road traffic accidents to professional negligence claims and employer liability disputes.
In Spencer v Wincanton Holdings Ltd [2009] EWCA Civ 1404, the claimant suffered a knee injury due to his employer’s negligence, which led to an above-knee amputation. Some time later, he fell at a petrol station while manoeuvring on his prosthetic leg and suffered further injury. The employer argued that the fall was a novus actus. The Court of Appeal disagreed. Sedley LJ held that the claimant’s decision to fill his own car with petrol was entirely reasonable. The fall was a foreseeable consequence of the disability caused by the original tort, and the chain of causation was not broken.
This decision confirms that courts adopt a generous view of what constitutes reasonable behaviour by an injured claimant. The law does not require claimants to live in a protective bubble. Provided the claimant’s conduct falls within the range of what a reasonable person in their position might do, the original defendant remains on the hook.
The interaction with contributory negligence
It is important to distinguish novus actus interveniens from contributory negligence. Where the claimant’s conduct contributes to the harm but does not break the chain of causation, the court may instead reduce the damages under the Law Reform (Contributory Negligence) Act 1943. Contributory negligence is a partial defence; novus actus interveniens, by contrast, is a complete defence that extinguishes the defendant’s liability for the subsequent harm. The distinction is often one of degree rather than kind, and the boundary between the two can be blurred in practice.
Professional negligence and successive tortfeasors
Modern commercial litigation frequently involves chains of professional advice in which multiple advisers contribute to a claimant’s loss. Where a subsequent adviser’s negligence intervenes between the original adviser’s breach and the claimant’s damage, the question of novus actus interveniens arises in a professional negligence context.
Courts have generally been reluctant to allow the first tortfeasor to escape liability simply because a later professional also acted negligently. The reasoning echoes the medical negligence cases: ordinary professional negligence by a subsequent adviser is unlikely to break the chain, because the involvement of further professionals is typically foreseeable. Only truly extraordinary conduct – conduct that no reasonable professional would have engaged in – is likely to qualify as a novus actus. This area intersects with the principles of assumption of responsibility and negligent misstatement.
Novus actus interveniens in criminal law
Although this guide focuses primarily on tort, the doctrine also applies in criminal law when determining whether the defendant’s act caused the prohibited result. The principles are broadly similar, but the criminal courts apply them with particular rigour given the consequences of a conviction.
In R v Smith [1959] 2 QB 35, a soldier stabbed a fellow soldier, who was then dropped twice while being carried to the medical station and received treatment described as “thoroughly bad.” The victim died. The Court Martial Appeal Court upheld the murder conviction, holding that the original wound was still the operating and substantial cause of death. The negligent treatment did not break the chain of causation.
By contrast, in R v Jordan (1956) 40 Cr App R 152, the victim of a stabbing was administered an antibiotic to which he was known to be allergic, causing his death. The Court of Criminal Appeal quashed the conviction, holding that the treatment was “palpably wrong” and constituted a novus actus interveniens. This case is often treated as exceptional on its facts, but it illustrates that the threshold for breaking the chain in criminal law, while high, is not insuperable.
The interaction between the defendant’s act and subsequent medical treatment was further considered in R v Cheshire [1991] 1 WLR 844, where the Court of Appeal held that negligent medical treatment would only break the chain if it was “so independent of [the defendant’s] acts, and in itself so potent in causing death,” that the jury could regard the defendant’s acts as insignificant. This sets an extremely demanding threshold, reflecting the criminal law’s concern to hold violent offenders responsible for the consequences of their actions.
Practical considerations and evidential burden
Who bears the burden?
The burden of establishing a novus actus interveniens rests on the defendant. Since the claimant will have established a prima facie causal link between the defendant’s wrong and the harm suffered, it falls to the defendant to demonstrate that the chain was broken by a subsequent intervening event.
In practice, this means the defendant must adduce evidence identifying the intervening act, establishing its independence from the original wrong, and demonstrating that it – rather than the defendant’s breach – was the effective cause of the harm. Expert evidence, particularly forensic medical evidence, is often crucial in this exercise. As one practitioner guide notes, detailed medical analysis may be required to disentangle the respective contributions of the original wrong and the intervening event.
Strategic considerations
From a litigation perspective, novus actus interveniens arguments carry both risk and reward. If successful, they provide a complete defence to liability for the subsequent harm. But they require the defendant to confront the factual detail of the intervening event head-on, which can sometimes strengthen the claimant’s narrative of overall harm. Defendants must weigh whether a novus actus argument is likely to succeed, or whether a contributory negligence argument – which concedes some liability but limits the quantum – would be the more prudent course.
Key principles summarised
| Factor | Explanation |
|---|---|
| Foreseeability | If the intervening act was reasonably foreseeable, it is unlikely to break the chain |
| Voluntariness | A free, deliberate, and informed act is more likely to constitute a novus actus |
| Independence | The act must be genuinely separate from the consequences of the original wrong |
| Reasonableness | Unreasonable conduct by the claimant may break the chain; reasonable responses will not |
| Threshold for medical negligence | Negligent treatment must be “so grossly negligent as to be completely inappropriate” – though this threshold is under review |
| Burden of proof | The defendant bears the burden of establishing that a novus actus has occurred |
Conclusion
Novus actus interveniens occupies a critical position within the law of causation. It represents the law’s recognition that liability must have boundaries, and that defendants should not be held responsible for harm that is truly attributable to an independent, supervening cause. At the same time, courts apply the doctrine with considerable care, mindful that too ready an acceptance of novus actus arguments would leave deserving claimants without a remedy.
The case law reveals a consistent theme: the more foreseeable the intervening act, and the more closely connected it is to the risk created by the defendant’s original wrong, the less likely it is to break the chain. Conversely, a truly independent, unforeseeable, and voluntary intervention – whether by the claimant, a third party, or a natural event — may relieve the defendant of liability for the subsequent consequences.
This area of the law continues to evolve. The Jenkinson decision raises fresh questions about the threshold for medical negligence as a novus actus, and modern litigation regularly tests the boundaries of the doctrine in new factual contexts. What remains constant is the courts’ commitment to a principled but pragmatic approach – one that seeks to do justice between the parties while maintaining coherent and workable rules of causation.
See also: Novus actus interveniens cases
References and further reading
- Marnham, M. (2023) ‘Novus Actus Interveniens: a critical analysis of Jenkinson v Hertfordshire CC’, Lexology, 2 May. Available at: https://www.lexology.com/library/detail.aspx?g=5c3fa42e-dd46-47f1-a7ce-9f3317b0456a
- Supreme Today AI (2025) ‘Deconstructing the Concept of Novus Actus Interveniens vis-à-vis the Chain of Causation’, 31 March. Available at: https://supremetoday.ai/doc/published-articles/deconstructing_the_concept_of_novus_actus_interveniens_vis_%C3%A0_vis_the_chain_of_causation
- Atkins, H. (2024) ‘Novus actus interveniens — A break in the chain of causation: consequences in personal injury cases’, Lyons Davidson Solicitors, 4 April. Available at: https://www.lyonsdavidson.co.uk/novus-actus-interveniens-a-break-in-the-chain-of-causation-consequences-in-personal-injury-cases/
- Halsbury’s Laws of England (2024) ‘Breaks in the chain of causation: novus actus interveniens’, Damages, vol 29, para 368. Available at: https://www.lexisnexis.co.uk/legal/commentary/halsburys-laws-of-england/damages/368-breaks-in-the-chain-of-causation-novus-actus-interveniens
Cite this work:
To cite this resource, please use the following reference:
National Case Law Archive, 'Novus actus interveniens in English law' (LawCases.net, March 2026) <https://www.lawcases.net/guides/novus-actus-interveniens-in-english-law/> accessed 29 April 2026
