Knightley v Johns [1982] 1 WLR 476 (CA)

Facts

A police officer negligently caused a tunnel to be partially blocked. A second officer was sent to direct traffic but gave instructions that led to the claimant being involved in a motorcycle accident. The claimant argued that the original negligent act of creating the obstruction caused his injuries.

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Issue

Can a subsequent act of a third party (here, the officer giving instructions) break the chain of causation, relieving the original defendant of liability?

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Decision

The Court of Appeal held that the original defendant was liable. The intervening act (incorrect instructions) was reasonably foreseeable in the circumstances of the tunnel blockage. Liability was not broken because the sequence of events flowed naturally from the original negligence.

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Legal Principle

A chain of causation is not broken by reasonably foreseeable acts, even if performed by third parties. Only independent, unforeseeable acts break the chain.

Courts assess whether the intervening act was a natural consequence of the defendant’s breach.

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Importance

• Clarifies limits of novus actus interveniens for third-party acts.

• Often cited in traffic, emergency, and workplace negligence cases.

• Helps distinguish between foreseeable and unforeseeable intervening causes.