Novus Actus Interveniens

Introduction

Novus actus interveniens (a new intervening act) is a doctrine used to decide whether an event occurring after the defendant’s conduct has broken the causal chain. If the chain is broken, the defendant is not legally responsible for the later harm.

The question is not simply what happened first and what happened next. The court asks whether the intervening event was independent, voluntary, and sufficiently significant to displace the original wrongdoing as the legal cause of the damage or death. The tests are applied in both criminal law and negligence, with close attention to whether the later act was a reasonable response, whether it was foreseeable, and whether it was a free and informed choice.

What You’ll Learn

• What novus actus interveniens means and why causation can be broken
• How voluntariness, informed choice, and foreseeability are assessed
• When a victim’s actions will and will not break the chain
• How third-party conduct (including police and rescuers) affects causation
• When natural events qualify as intervening acts
• The role of medical treatment and omissions (including refusal of treatment)
• Key differences and common ground between criminal law and negligence
• Leading UK cases that shape the tests and their practical use

Core Concepts

What breaks the chain?

• The chain of causation is broken when a later event is independent of the defendant’s act and so significant that it becomes the legal cause of the harm.
• Courts distinguish between:
  • A reasonable reaction to the defendant’s conduct (usually does not break the chain), and
  • A free, voluntary and informed act by another person that introduces a new cause (often breaks the chain).
• In both criminal and civil cases, foreseeability and reasonableness are key signposts. If a reaction was a predictable response to the danger created by the defendant, the chain is less likely to be broken.

Voluntary and informed choices

• A free, voluntary and informed decision by an adult of sound mind can break the chain. Example: self-injection of drugs knowingly and freely taken.
• Where the person’s will is overborne by fear, pressure, or the circumstances created by the defendant, the act may not be treated as truly voluntary.
• Context matters: decisions made under extreme pain, disfigurement or severe pressure are less likely to be seen as acts of “free and unfettered volition”.

Reasonableness and foreseeability

• A reasonable response to danger (self-defence, rescue, or acting in the course of duty) typically does not break the chain, even if it contributes to the harm.
• An unreasonable or unforeseen act may break the chain, particularly where the claimant or victim makes a choice that goes well beyond what was a natural or probable consequence of the original wrong.
• In negligence, the threshold to treat a claimant’s conduct as breaking the chain is high; courts avoid penalising ordinary misjudgments made in difficult conditions.

Natural events (Acts of God)

• Natural events can break the chain if they are extraordinary and not reasonably foreseeable in the circumstances.
• Ordinary weather or seasonal events, even if inconvenient or heavy, are unlikely to qualify. Severe and unexpected storms may do so.

Medical treatment and omissions

• Medical treatment following an injury is usually foreseeable. Negligent treatment rarely breaks the chain unless it is so bad that it becomes a new cause of death or injury.
• A victim’s refusal of medical treatment will not normally break the chain if the original wound remains an operating and substantial cause of death. The thin skull rule applies: take your victim as you find them, including their beliefs.

Victims, rescuers, and third parties

• Acts by rescuers or those performing legal duties (such as police officers) are frequently treated as reasonable responses to the danger created by the defendant.
• Third-party negligence can sometimes break the chain if it is sufficiently independent and unforeseeable, but ordinary mistakes in the heat of the moment are often treated as part of the overall sequence caused by the defendant.
• In negligence, a later negligent act can break the chain where it introduces a new, dominant cause, especially if the original wrongdoing had run its course.

Key Examples or Case Studies

• R v Kennedy (No 2) [2007] UKHL 38

  • Context: Defendant supplied heroin; the victim self-injected and died.
  • Holding: The victim’s free and informed decision to self-inject broke the chain. The supplier was not liable for unlawful act manslaughter.
  • Use: Look for voluntary, informed choices by adults.

• R v Pagett (1983) 76 Cr App R 279

  • Context: The defendant used a hostage as a shield; police returned fire, causing the hostage’s death.
  • Holding: The police shooting was a reasonable act in self-defence and did not break the chain.
  • Use: Reasonable actions in self-defence or duty are often part of the causal sequence.

• R v Wallace [2018] EWCA Crim 690

  • Context: Victim, disfigured by an acid attack, later chose euthanasia abroad.
  • Holding: The decision was not a free and unfettered choice in the legal sense; the chain was not broken.
  • Use: Choices made under extreme pressure from injuries may not count as voluntary for causation.

• McKew v Holland & Hannen [1969] 3 All ER 1621

  • Context: Claimant with a weakened leg attempted steep stairs unaided and was injured.
  • Holding: The claimant acted unreasonably; his conduct broke the chain.
  • Use: Unreasonable conduct by a claimant can interrupt causation in negligence.

• Spencer v Wincanton Holdings [2009] EWCA Civ 1404

  • Context: Further injury occurred after an earlier workplace accident.
  • Holding: The claimant’s conduct did not reach the high threshold of unreasonableness needed to break the chain.
  • Use: Courts apply a high bar before blaming claimants for later mishaps.

• Carslogie Steamship Co Ltd v Royal Norwegian Government [1952] AC 292

  • Context: Ship damaged in a collision later suffered storm damage at sea.
  • Holding: The severe storm was a novus actus interveniens; the defendant was liable only for the collision damage.
  • Use: Extraordinary natural events can sever causation.

• Southern Water Authority v Pegrum [1989] Crim LR 442

  • Context: Heavy rain led to overflow and pollution.
  • Holding: Heavy rain was foreseeable and not an Act of God; no break in the chain.
  • Use: Ordinary weather conditions are rarely treated as intervening acts.

• R v Blaue [1975] 1 WLR 1411

  • Context: Stabbing victim refused a blood transfusion for religious reasons and died.
  • Holding: No break in the chain. Thin skull rule applied; the original wound remained the operating cause.
  • Use: Respect for a victim’s beliefs does not relieve the defendant of liability.

• Knightley v Johns [1982] 1 WLR 349

  • Context: After a crash caused by the defendant, a senior officer negligently ordered an officer to ride against traffic, causing injury.
  • Holding: The negligent order was a new intervening act that broke the chain from the initial crash.
  • Use: Independent, negligent third-party decisions can displace the original cause.

Practical Applications

• Map the sequence clearly

  • Identify the initial wrongful act and each subsequent event.
  • Ask at each step: is the later event a natural, reasonable response, or does it introduce a new cause?

• Test for voluntariness and informed choice

  • Was the person an adult of sound mind?
  • Was the decision free from fear, coercion, or pressure created by the original wrongdoing?
  • If yes, treat this as a strong candidate for breaking the chain (e.g., self-injection).

• Assess reasonableness and foreseeability

  • Was the response (self-defence, rescue, performance of duty) objectively reasonable?
  • Would a reasonable person expect this reaction to follow from the defendant’s conduct?
  • Reasonable and foreseeable reactions usually keep the chain intact.

• Consider medical treatment and omissions

  • Ordinary or even negligent treatment usually does not break the chain.
  • Only treatment that is so poor that it becomes a new cause will interrupt causation.
  • A victim’s refusal of treatment generally does not break the chain if the original injury is still operating.

• Distinguish criminal and negligence contexts

  • Criminal: the prosecution must prove causation beyond reasonable doubt. Focus on whether the defendant’s act remained an operating and substantial cause.
  • Negligence: claimant must prove causation on the balance of probabilities. The bar for treating a claimant’s conduct as breaking the chain is high; ordinary misjudgments are not enough.

• Apply the thin skull rule

  • Take the victim as you find them—physical condition, beliefs, and vulnerabilities. Do not treat these characteristics as breaking the chain.

• Structure a problem answer

  1. Identify the type of intervening act: victim, third party, natural event, or medical treatment/omission.
  2. Apply the tests: independence, voluntariness, reasonableness, and foreseeability.
  3. Compare with leading cases in the same category (e.g., Kennedy for voluntary self-injection, Pagett for self-defence).
  4. Conclude with a clear statement on whether the chain is broken and why.

Summary Checklist

• Define novus actus interveniens: a new, independent event that displaces the defendant’s conduct as the legal cause.
• Voluntary, informed adult choices can break the chain; pressured or constrained choices usually do not.
• Reasonable and foreseeable reactions (self-defence, rescue, legal duty) typically keep the chain intact.
• Unreasonable claimant conduct can break the chain in negligence, but the threshold is high.
• Extraordinary, unforeseeable natural events can break the chain; ordinary weather does not.
• Medical treatment rarely breaks the chain unless it is so poor that it becomes a new cause.
• Refusal of treatment usually does not break the chain; apply the thin skull rule.
• Anchor your analysis with cases: Kennedy, Pagett, Wallace, McKew, Spencer, Carslogie, Pegrum, Blaue, Knightley.

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