The 'but for' test: proving factual causation in professional negligence

Reviewed by Matthew Bartlett, Director · Last reviewed 2026-07-01

Factual causation is the bridge between a professional's negligence and the loss a client claims to have suffered. Without it, a claim in professional negligence cannot succeed, however clearly the duty and the breach are made out. The starting point in English law is the 'but for' test: would the loss have occurred but for the professional's negligent act or omission? If the answer is that the loss would have happened anyway, the negligence is not the factual cause and the claim fails at this hurdle.

The classic 'but for' test

The test is often illustrated by Barnett v Chelsea & Kensington Hospital Management Committee [1969] 1 QB 428. A watchman attended casualty complaining of vomiting; the doctor on duty negligently refused to examine him and sent him home. He died a few hours later of arsenic poisoning. The court found the hospital had breached its duty, but the claim failed on causation: medical evidence showed the poisoning was so advanced that competent treatment would not have saved him. The negligence was not, on the balance of probabilities, the factual cause of death.

The same logic runs through professional negligence. The claimant must prove, on the balance of probabilities, that competent advice or conduct would have produced a different outcome. If the outcome would have been the same regardless, the professional's breach does not sound in damages.

Where the courts relax the test

The strict 'but for' test does not always sit comfortably with cases involving multiple potential causes, particularly in industrial-disease litigation. In Bonnington Castings Ltd v Wardlaw [1956] AC 613, the House of Lords accepted that where an injury arises from cumulative exposure to a harmful agent, it is enough to show the defendant's breach made a material contribution to the harm.

The principle was extended in Fairchild v Glenhaven Funeral Services Ltd [2002] UKHL 22, a mesothelioma case in which the claimant had been exposed to asbestos by multiple employers and could not identify which exposure triggered the disease. The House of Lords held that material contribution to the risk of harm was sufficient. A related relaxation appeared in Chester v Afshar [2004] UKHL 41, where a surgeon's failure to warn of a small risk was treated as causative of the injury that followed.

These exceptions are narrow. In ordinary professional negligence claims against solicitors, accountants or financial advisers, the courts apply the orthodox 'but for' test.

Practical application in professional indemnity claims

The counterfactual analysis is central. The claimant must set out what a competent professional would have done, and then persuade the court that this would have led to a different, better outcome. Common scenarios include:

• A solicitor who advises late on a limitation deadline: would the client have issued proceedings in time had proper advice been given, and would those proceedings have succeeded?
• An independent financial adviser who negligently recommends a defined-benefit pension transfer: would the client have declined had the risks been properly explained?
• An accountant who misses a tax filing deadline: would the client have submitted on time had a competent reminder system been in place?

In transaction cases, the courts often frame the question as the 'no transaction' counterfactual: would the client have proceeded with the deal, investment or step at all had competent advice been given? This intersects with the scope-of-duty analysis under the SAAMCo principle, and with the standard of care governed by the Bolam test.

Worked example (hypothetical)

Worked example: An IFA recommends that a client, aged 58, transfer her defined-benefit pension into a self-invested personal pension. The IFA fails to explain the loss of guaranteed inflation-linked income, the transfer-value trade-off, and the market risk on the transferred fund. The transfer proceeds; the fund falls in value; the client sues.

To succeed on factual causation, the client must show that, on the balance of probabilities, she would not have transferred had she been advised properly. Supporting evidence might include contemporaneous notes recording a cautious risk profile, correspondence expressing a preference for guaranteed income, or a witness statement confirming she would have declined. Conversely, if the evidence shows a family member was pressuring her to transfer so the funds could be released to the wider family, and the transfer would have gone ahead regardless, the 'but for' test is not satisfied. The negligence, however clear, is not the factual cause of the loss and the claim fails at causation.

Factual causation is often the most contested issue in professional negligence litigation. It rewards contemporaneous documentation and disciplined file-keeping, and it can be the difference between a claim that settles and one that fails at trial.

Apex Insurance Brokers Limited is authorised and regulated by the Financial Conduct Authority. Firm reference number 724952. This entry is general information, not advice on any particular policy.

Apex Insurance Brokers Limited is authorised and regulated by the Financial Conduct Authority. Firm reference number 724952.