What is “the defence of illegality” and when does it apply?

Who doesn’t love a Latin maxim?

This article is about a particular one - ex turpi causa non ortitur actio. When it applies, a complete defence to a claim is made out. The maxim is sometimes referred to as “the defence of illegality” but it has also been translated, in UK judicial decisions, into different versions of English words, including -  

• “No action arises from a disgraceful cause.”
• “No action arises from an immoral transaction.”

A question arises on whether “disgraceful” or “immoral”, for these purposes, must mean “criminal”. A further question arises on the relevance, or otherwise, of the extent to which the conduct or circumstances is connected to what went wrong in leading to the claim. 

As we will see, in deciding whether “the defence of illegality” is made out, these questions have tested courts UK-wide and continue to do so.

Before looking at certain specific cases, it is always good to start at the top by considering certain overarching points made by the United Kingdom Supreme Court (UKSC).

One “essential rationale”, two “broad discernible policy reasons” and three “necessary considerations”

Giving the majority judgment of a bench of nine justices at the UKSC in Patel v Mirza [2017] A.C. 467, Lord Toulson made these overarching points (with emphasis added) -

1. “The essential rationale of the illegality doctrine is that it would be contrary to the public interest to enforce a claim if to do so would be harmful to the integrity of the legal system (or, possibly, certain aspects of public morality, the boundaries of which have never been made entirely clear and which do not arise for consideration in this case).”
2. “… there are two broad discernible policy reasons for the common law doctrine of illegality as a defence to a civil claim. One is that a person should not be allowed to profit from his own wrongdoing. The other, linked, consideration is that the law should be coherent and not self-defeating, condoning illegality by giving with the left hand what it takes with the right hand.”
3. “… one cannot judge whether allowing a claim which is in some way tainted by illegality would be contrary to the public interest, because it would be harmful to the integrity of the legal system, without a) considering the underlying purpose of the prohibition which has been transgressed, b) considering conversely any other relevant public policies which may be rendered ineffective or less effective by denial of the claim, and c) keeping in mind the possibility of overkill unless the law is applied with a due sense of proportionality. We are, after all, in the area of public policy. That trio of necessary considerations can be found in the case law.”

Let us now look at certain specific cases to see how these points have been applied to particular facts and circumstances.

Interactions with police upon arrest

The key background facts, and findings in fact and law, in the most recent Scottish case on “the defence of illegality”, Dawson v Threadgold and others 2025 G.W.D. 30-288, were -

• The pursuer was convicted of the criminal offences of failing to - (i) have a valid MOT certificate, (ii) update details on his driving licence, (iii) allow police to examine his vehicle, and (iv) provide his name and address to police.
• Two police officers acted reasonably in removing the pursuer from his vehicle, transferring him to the bonnet of a police vehicle and handcuffing him.
• The police officers acted unreasonably in then each grabbing an arm of the pursuer and letting go, causing him to fall to the ground to his injury.

The key question in deciding whether the police were liable to pay damages to the pursuer was whether the illegality on the part of the pursuer gave the police a defence. 

The sheriff decided that the defence of illegality was not made out, so the police were liable to pay damages for the injury caused when the pursuer fell to the ground. The sheriff’s reasons for this were that “The criminal activity on the part of the pursuer was minor. It could never reasonably be categorised as significant, and it was not integral to the negligence on the part of the defenders' use of unreasonable force. There is a clear break in the chain of causation between the pursuer being arrested and him being allowed to fall to the ground and sustain injury.”

So, the level of criminality and the extent of its causative connection to the acts that caused injury were the key factors in the defence of illegality not being established. The “trio of necessary considerations” outlined above can be seen here, especially (b) in the context of public policy against unreasonable use of force by the police, and (c) in the context of proportionality in considering causation and on the level of criminality. 

Being intentionally struck by a car while attacking premises, potentially with a view to attacking people within them

In the Scottish case McLaughlin v Morrison 2014 S.L.T. 111, the pursuer was not convicted of crime but it was claimed that he was part of a group attacking premises, potentially with a view to attacking people within them, when the defender intentionally drove into him (it seems with a view to stopping the attack), to his severe injury. 

Again, a focus on causation, that can be analysed as part of the proportionality consideration, led the court to hold that the defence of illegality was not established. 

The court reasoned that “it cannot be said that, although the damage would not have happened but for the alleged illegal conduct of the defender, it was caused by the criminal act of (the pursuer). The position is, rather, that although the damage would not have happened without the alleged criminal act of (the pursuer), it was caused by the illegal act of the defender.”

Allowing oneself to be driven by someone known not to have a driving licence

In another Scottish case, Weir v Wyper 1992 S.L.T. 579 (link not publicly available), the pursuer, aged 16, was seriously injured by the negligent driving of an unqualified driver who she allowed herself to be a passenger of. The defender argued that this meant she was barred from recovering damages because she took part in a common criminal activity. 

Again, for proportionality and policy reasons, the court held that the defence of illegality was not established. In deciding this, the court focused on the pursuer not participating in any significant criminal activity and that any reasonable application of public policy would allow her to recover damages. 

Allowing oneself to be driven by someone known not to have a driving licence, known to be intoxicated and encouraging them to drive recklessly

Onto an English case now, Pitts v Hunt [1991] 1 Q.B. 24. This case can be contrasted with Weir in that, in Pitts, not only did the claimant know that the motorcycle driver, who he was riding as a pillion passenger with, was unlicensed, he also knew that the driver was intoxicated, them both having been drinking alcohol together, and he also encouraged the driver to drive recklessly. In these circumstances and in the context of the claimant suing for injuries sustained in a collision between the motorcycle and a car, the court held that the defence of illegality was met such that the claimant was barred from recovering any damages. 

The key point, in the court’s analysis, was that the claimant’s involvement in the unlawful conduct was not incidental but central to the incident causing injury. Again, we can see the overarching points summarised above at play here - as a matter of policy, the law will block recovery where criminality is positively encouraged by the claimant where that criminality is causally significant to the incident causing injury.  

Schizophrenic suing for damages, and seeking indemnity, for being at large when he killed three men after being found not guilty of murder by reason of insanity

The litigation in Lewis-Ranwell heard at UKSC on 15 and 16, both July 2025 shows that the nature and extent of the defence of illegality continues to test the courts. The claimant sues certain authorities or organisations for not having detained him before, when still “at large” in the public, he killed three men. 

In response to the defendants’ attempts to bar the claimant from proceeding on the basis of the defence of illegality, both the High Court and, by majority, the Court of Appeal determined that because the claimant’s plea of insanity succeeded at the criminal trial, he did not know that the killings were wrongful, so the defence was not established. UKSC is now considering whether this decision is correct in law.

Aside from the specific point on knowledge of wrongdoing, UKSC may take the opportunity to give wider guidance on the nature and extent of the defence of illegality such that their judgment, which remains pending, may influence thinking and analysis in other cases with very different facts. 

Conclusion

We have seen that outcomes in cases where the defence of illegality is argued are intensely fact specific. We have also seen that the guiding points of law in this area are quite generally expressed. More specific guidance might follow soon from UKSC but cases in this area are likely to still test both lawyers and the courts in the months and years to come.