Category: Insurance case law · Reviewed by Mark Fox, Broker · Renewals · Last reviewed June 2026
A leading Court of Appeal decision on novus actus interveniens in motor accident chains of causation, holding that a deliberate negligent act by a third party broke the chain of causation between the original driver’s negligence and a subsequent injury.
[verify Bailii URL — pre-2001 Court of Appeal decisions not always on Bailii][verify exact date]On 6 February 1976, the first defendant, Mr Johns, negligently drove his car in the Queensway road tunnel in Birmingham and overturned the vehicle near the exit of the tunnel. Police officers attended the scene. A senior officer at the tunnel exit failed to close the tunnel immediately to oncoming traffic, contrary to standing police orders which required tunnel closure in such situations. Realising the omission, the senior officer instructed the claimant, Police Constable Knightley, and another officer to ride their motorcycles back through the tunnel against the flow of oncoming traffic to close the entrance at the far end. While carrying out that instruction, Constable Knightley was struck by an oncoming car driven by the second defendant and sustained serious injuries. The claimant sued the original negligent driver (Johns), the chief constable in respect of the senior officer’s negligent instruction, and the oncoming driver. The trial judge found that the senior officer had been negligent in failing to close the tunnel promptly and in instructing the officers to ride against the traffic, but apportioned liability between the original driver Johns and the police, treating the original negligence as still operative. Johns appealed, contending that the senior officer’s intervening conduct was so unreasonable that it broke the chain of causation from his original negligence. The Court of Appeal had to determine whether the original tortfeasor’s liability extended to the injury suffered by the claimant or whether the intervening acts amounted to a novus actus interveniens.
The principal issue was whether the negligence of the senior police officer in failing to close the tunnel and in giving the dangerous instruction to ride back through it amounted to a novus actus interveniens, breaking the chain of causation between the original negligent driver and the injury sustained by Constable Knightley. The court had to consider how to characterise an intervening act by a third party: whether the test was foreseeability of the kind of intervening act, whether negligence of the intervening party was sufficient to break the chain, and whether the original tortfeasor remained liable for injuries indirectly caused. The wider issue was the conceptual basis on which English law allocates responsibility where a sequence of negligent acts and decisions produces a harm distant in time and circumstance from the original wrong.
The Court of Appeal (Stephenson LJ giving the leading judgment) allowed Johns’s appeal and held that the senior officer’s conduct constituted a novus actus interveniens which broke the chain of causation from the original negligence. The court considered that the senior officer’s failure to close the tunnel, and his instruction to the officers to ride against the traffic, were a series of mistakes and a deliberately dangerous act sufficiently grave to be characterised as new causes rather than as part of the ordinary consequences of Johns’s original negligence. Stephenson LJ analysed causation in terms of what was natural and probable in the context, distinguishing between reasonable errors made in the immediate “agony of the moment” (which would not break the chain) and deliberate, considered acts of negligence such as those of the senior officer. The court emphasised that whether an intervening act breaks the chain is “a question of fact and degree” to be determined by applying common sense rather than by mechanical rules. Liability for the claimant’s injuries accordingly rested with the police authority (and with the oncoming driver to the extent attributable to him), and Johns was relieved of liability for the consequences that followed the senior officer’s intervening conduct.
Where a sequence of events follows an original negligent act, the original tortfeasor remains liable only for those consequences which are the natural and probable result of the original negligence. A subsequent negligent or unreasonable act by a third party may amount to a novus actus interveniens, breaking the chain of causation, where it is sufficiently unreasonable, deliberate or independent of the original wrong. Whether an intervention breaks the chain is a question of fact and degree, to be answered by applying common sense and recognising the distinction between instinctive responses to an emergency and considered acts of negligence after the immediate danger has passed.
Knightley v Johns is a foundational authority for motor and casualty insurers on causation issues in multi-party liability claims. For motor insurers, the decision is relevant whenever a road traffic accident triggers a chain of events involving emergency services, rescuers, secondary collisions or onward complications. The decision establishes that insurers of an original tortfeasor are not necessarily liable for every downstream consequence: where an independent and unreasonable intervening act occurs, that downstream loss may be attributed to a different party and a different insurer. For claims handlers, the case is routinely cited where multiple parties are joined and the question is how the indemnity burden should be allocated between motor, public liability, employers’ liability and other insurers. The case also informs subrogated recovery actions, where insurers paying out for losses arising from a complex chain of events need to assess whether their insured’s negligence was a continuing operative cause of all the losses claimed or only some. For brokers placing motor fleet, emergency services and public liability covers, Knightley is a reminder of the importance of considering causation when assessing exposure, structuring policy limits and discussing aggregate limits with insurers. Apex Insurance Brokers references the decision in causation-related case studies for motor and public liability clients.
By Matt Bartlett, Director, on 2026-06-06. Next review: 2026-12-06.
This entry is part of the Apex Insurance Wiki. Last reviewed by Matt Bartlett on 2026-06-06. Apex Insurance Brokers Limited, FCA FRN 724952, Companies House 07014570. Not regulated advice — consult your broker on your specific position.
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