Kleinwort Benson Ltd v. Lincoln City Council Kleinwort; Benson Ltd v. Mayor etc of the London Borough of Southwark and Others Kleinwort; Benson Ltd v. Birmingham City Council Mayor etc of the London Borough of Kensington and Chelsea and Others

Factual and Procedural Background

The Appellant, a merchant bank, entered into a series of interest-rate swap agreements with several English local authorities (the Respondents) during the 1980s. In Hazell v. Hammersmith and Fulham LBC the House of Lords subsequently ruled that such swap transactions were ultra vires local authorities and therefore void ab initio. The Appellant sought restitution of net payments totalling approximately £423,000 that had been made more than six years before the writs were issued. Because those payments lay outside the standard six-year limitation period for simple contract claims, the Appellant advanced an alternative cause of action in unjust enrichment, contending that the sums were paid under a mistake of law.

Judge Langley ordered the trial of preliminary issues. He held himself bound by Court of Appeal authority to reject recovery for mistake of law and certified a leap-frog appeal. The House of Lords therefore had to decide, inter alia, whether the long-standing common-law bar on recovery of money paid under a mistake of law should survive.

Legal Issues Presented

1. Whether the historic rule that money paid under a mistake of law is irrecoverable should remain part of English law.
2. If recovery for mistake of law is recognised, whether a special defence exists where the payment was made in accordance with a “settled understanding” of the law later overturned by judicial decision.
3. Whether an “honest receipt” defence (i.e., the payee honestly believed the money was theirs to keep) bars recovery in mistake-of-law cases.
4. Whether full performance of a void swap contract (a “closed” swap) prevents restitutionary recovery on the ground of mistake.
5. Whether section 32(1)(c) of the Limitation Act 1980 (postponement for discovery of a mistake) applies to mistakes of law.

Arguments of the Parties

Appellant’s Arguments

• The mistake-of-law bar lacks principled justification and is inconsistent with the modern law of unjust enrichment.
• International and Commonwealth authorities, plus scholarly criticism, favour abolition; English law should align with that consensus.
• The proposed “settled understanding” and “honest receipt” defences are unnecessary because existing doctrines—especially change of position, compromise, estoppel and limitation— already protect transactional security.
• The fact that swap contracts were fully performed is irrelevant; each payment is a separate unjust enrichment if made under mistake.
• Section 32(1)(c) expressly covers “mistake” without limiting words and therefore postpones time-running in mistake-of-law claims.

Respondents’ Arguments

• The House should leave abolition or reform of the rule to Parliament so that limitation and other safeguards can be legislated simultaneously.
• If the rule is abolished judicially, recovery must be curtailed by a defence where the payment was made pursuant to a settled, widely-held understanding of the law subsequently revised (“settled law” defence).
• An additional defence should arise where the defendant honestly believed, on learning of the payment, that it was entitled to retain the money (“honest receipt” defence).
• Completed swaps should not be unwound: once both parties have fully performed, no restitutionary claim should lie.
• Section 32(1)(c) was never intended to cover mistakes of law; applying it would prolong uncertainty and litigation.

Table of Precedents Cited

Court’s Reasoning and Analysis

1. Mistake-of-law rule: The House (per Judge Goff) held that the bar is incompatible with the organising principle of unjust enrichment, produces arbitrary distinctions, and has been rejected in most major common-law jurisdictions. Historical reliance on the maxim “ignorance of the law is no excuse” was dismissed as doctrinally unsound.

2. Settled-law and honest-receipt proposals: The majority declined to create these special defences judicially. A “settled understanding” test would be vague, fact-intensive and unnecessary given existing protections; an “honest receipt” defence would be so broad that it would swallow restitutionary claims and conflict with the established change-of-position doctrine.

3. Completed swaps: Full performance of a void contract does not prevent restitution. Each net payment constitutes a separate unjust enrichment claim arising at the moment of receipt. To hold otherwise would undermine the ultra vires doctrine by leaving an unlawful transaction effectively unchallenged.

4. Limitation Act 1980, s. 32(1)(c): The term “mistake” is unqualified; the equitable postponement rule therefore applies to mistakes of law. Time runs only from the date the mistake was discovered or could reasonably have been discovered—here, no earlier than the 1991 decision in Hazell.

Holding and Implications

Appeals Allowed.

The House of Lords abolished the common-law rule barring restitution for money paid under a mistake of law. The Appellant’s pleaded cause of action in mistake therefore lies, completed swap payments are recoverable in principle, and the postponement provision in section 32(1)(c) of the Limitation Act 1980 applies.

Implications: The decision aligns English law with other common-law jurisdictions, embeds unjust enrichment as the governing principle, and invites legislative review of limitation periods. Existing defences (change of position, compromise, estoppel) remain; no new, special defences were recognised. Public authorities and commercial actors must therefore assess historic payments in light of the broadened restitutionary remedy.