Novus actus – intervening acts as an element of causation

In the tort of negligence and medical malpractice in particular, it is often easier to establish a breach of duty than it is to establish a causal link between that breach and the injury or loss claimed. That difficulty is compounded where the medical malpractice comes after an event, such as a road accident or injury at work, that has injured a claimant and put them in hospital for treatment that is meant to resolve, cure or mitigate their injury, but makes the situation worse.

Both factual and legal causation determine whether a party is liable for the damage caused to another. Another element of causation, often overlooked and/or misunderstood, is that of novus actus interveniens.

Novus actus interveniens is Latin for a "new intervening act". A novus actus breaks the causal chain linking the tortfeasor’s action (the initial wrongdoer) to their liability to the injured person. To constitute a novus actus the secondary act cannot be reasonably foreseeable. That said, the intervening act can be occasioned by anyone or anything including the injured party themselves, another third party or an act of God.

Original tortfeasors may use novus actus to assert their liability is limited or non-existent and falls to another party. It can be distinguished from contributory negligence where the act precedes the act causing injury.

A recent illustration

In the case of Jenkinson v Hertfordshire County Council the claimant fractured an ankle in an uncovered manhole. The council admitted liability but argued that had the initial surgery undertaken by the hospital where the claimant was taken for treatment been carried out correctly, the claimant would have been able to return to work shortly afterwards, having largely recovered from his ordeal, and would not have had to undergo six further surgical interventions over the next three years. It argued the negligence of the hospital was a novus actus which broke the chain of causation.

The case law

On appeal from a decision to disallow an application to amend the defence, Baker J took the opportunity to review the case law around novus actus and causation. One of those cases was Webb v Barclays Bank plc and Portsmouth Hospitals NHS Trust (2002) where the claimant injured a knee at work. After that injury she was negligently advised to have an above knee amputation. The Court of Appeal subsequently held that the surgeon's negligent advice "did not eclipse the original wrongdoing" and determined that the chain of causation had not been broken. 

Successive acts of medical negligence

What about a situation where one medical team is negligent causing injury and then subsequent teams are also negligent? This was the argument in the case of Wright v Cambridge Medical Group (2011) where a GP referral to hospital was negligently delayed and where the GP argued that even if the referral had been made on time, the hospital team would have negligently failed to treat the patient leading to inevitable injury. In that case Lord Neuberger said:

“… where there are successive tortfeasors, the contention that the causative potency of the negligence of the first is destroyed by the subsequent negligence of the second depends very much on the facts of the particular case. In many cases where there are successive acts of negligence by different parties, both parties can be held responsible for the damage which ensues, so that the issue is not which of them is liable, but how liability is to be apportioned between them. The mere fact that, if the second party had not been negligent, the damage which subsequently ensued would not have occurred, by no means automatically exonerates the first party’s negligence from being causative of that damage.”

See also Widdowson's Executrix v Liberty Insurance Ltd (2021), a Scottish authority which again apportioned liability between original and successive tortfeasors following a road traffic accident and subsequent negligent omissions and treatment by two separate hospitals.

An unforeseeable event?

In his dissenting judgment in Hogan v Bentinck West Hartley Collieries (Owners) Ltd (1949), Lord Reid considered that only a "grave lack of skill and care" (which is to say something unforeseeable) in the provision of intervening medical treatment could serve to break the chain of causation.

Clerk & Lindsell on Torts considers Lord Reid's approach to be correct: "only medical treatment so grossly negligent as to be a completely inappropriate response to the injury inflicted by the defendant should operate to break the chain of causation."

This passage was specifically approved by the Court of Appeal in the case of Webb and accordingly the claimant's employers were held liable for all the damage attributable to the fall, and 25 per cent of the damage attributable to the amputation.

Novus actus as a tool in the toolbox

Intervening medical treatment was also considered in the case of Rahman v Arearose Ltd (2001). The claimant was assaulted whilst at work causing an eye injury. His employers were held liable for the assault. The subsequent negligence of his surgeon resulted in the clamant being rendered blind in the affected eye.

The claimant developed severe psychiatric consequences, partly due to the assault and partly due to the loss of his eye. Laws LJ stated:

"On these materials it does not seem to me to be established as a rule of law that later negligence always extinguishes the causative potency of an earlier tort. Nor should it be. The law is that every tortfeasor should compensate the injured claimant in respect of the loss and damage for which he should justly be held responsible. To make that principle good, it is important that the elusive conception of causation should not be frozen into constricting rules."

He continued:

"So in all these cases the real question is, what is the damage for which the defendant under consideration should be held responsible. The nature of his duty (here, the common law duty of care) is relevant: causation, certainly, will be relevant – but it will fall to be viewed, and in truth can only be understood, in light of the answer to the question: from what kind of harm was it the defendant's duty to guard the claimant?"

While not referring to “material contribution” directly, one must consider that to be among his thoughts when he said that:

"Novus actus interveniens, the eggshell skull rule, and (in the case of multiple torts) the concept of concurrent tortfeasors are all no more and no less than tools or mechanisms which the law has developed to articulate in practice the extent of any liable defendant’s responsibility for the loss and damage which the claimant has suffered."

A strict or specific rule, or no rule?

Coming back to the recent appeal in Jenkinson, Baker J noted that the judge below had considered the Webb case and held that it established as a rule of law that medical treatment of an injury caused by a defendant’s tort cannot break the chain of causation unless it is such grossly negligent treatment as to be a completely inappropriate response to the injury (“the Specific Rule”).

Summarising the law more generally on the intervening conduct of a third party, the judge referred to Clerk & Lindsell where it states that:

“No precise or consistent test can be offered to define when the intervening conduct of a third party will constitute a novus actus interveniens sufficient to relieve the defendant of liability for his original wrongdoing…. Four issues need to be addressed. Was the intervening conduct of the third party such as to render the original wrongdoing merely a part of the history of events? Was the third party’s conduct either deliberate or wholly unreasonable? Was the intervention foreseeable? Is the conduct of the third party wholly independent of the defendant, ie does the defendant owe the claimant any responsibility for the conduct of the intervening third party? In practice, in most cases of novus actus more than one of the above issues will have to be considered together.”

So in Jenkinson, Baker J held:

“In my judgment, the Specific Rule does not exist as a principle of law defining a necessary ingredient of a novus actus defence in the context of medical interventions.”


If there is a subsequent unforeseeable act that so eclipses the original event and injury such that it is reasonable and proper to identify a break in the chain of causation, you may have an argument to run. For the most part, subsequent medical negligence will probably be an “honest mistake” insufficient to be a novus actus, but likely to provide for some measure of apportionment of liability. It would be advisable to test your expert evidence on causation at an early stage.

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