Ex Turpi Causa

The maxim's classic formulation traces back over two centuries: the Supreme Court's judgment in Patel v Mirza records Lord Mansfield's statement in Holman v Johnson (1775) that no court will lend its aid to a man who founds his cause of action upon an immoral or an illegal act. For most of that history, English courts applied illegality as something close to a blanket bar wherever a claimant's case was tainted by unlawful conduct.

Patel v Mirza reformulated the modern test. Rather than a mechanical rule, the Supreme Court held that in assessing whether the public interest would be harmed by allowing a claim tainted by illegality, a court should stand back and consider whether the contemplated result would be contrary to the public interest, because it would be harmful to the integrity of the legal system, weighing this against the underlying purpose of the prohibition that was breached, any other public policies that might be undermined by refusing the claim, and the need to avoid disproportionate 'overkill' in denying relief.

This shift replaced a rigid, technical approach — under which the outcome could turn on whether the claimant needed to plead their own illegality to establish the claim — with a structured but flexible balancing exercise, giving courts considerably more discretion to allow a claim to proceed even where illegality is present, provided that refusing it would be a disproportionate response to the wrongdoing involved.