Unjust enrichment in English law

The English law of unjust enrichment is part of the English law of obligations, along with the law of contract, tort, and trusts. The law of unjust enrichment deals with circumstances in which one person is required make restitution of a benefit acquired at the expense of another in circumstances which are unjust.

The modern law of unjust enrichment encompasses what was once known as the law of quasi-contract. Its precise scope remains a matter of controversy. Beyond quasi-contract, it is sometimes said to encompass the law relating to subrogation, contribution, recoupment, and claims to the traceable substitutes of misapplied property.

English courts have recognised that there are four steps required to establish a claim in unjust enrichment. If the following elements are satisfied, a claimant has a prima facie right to restitution:

The law of unjust enrichment is among the most unsettled areas of English law. Its existence as a separate body of law was only explicitly recognised in 1991 in Lipkin Gorman v Karpnale Ltd. While the law has rapidly developed over the last two decades, controversy continues over the precise structure, scope and nature of the law of unjust enrichment.

The notion of an obligation to make restitution of benefits received at another's expense can be traced back to Roman law. Its history in English law can be traced to the form of action known as indebitatus assumpsit. From this action came the 'common money counts'. Of present relevance are the following:

During the course of the 18th and 19th centuries the law of contract, the law of tort and the law of trusts emerged as discrete bodies of law within English private law. As many thought they covered the field, restitutionary claims as embodied in the common money counts were appended to the law of contract. The various claims were termed 'quasi-contractual'. This was because it was often necessary to plead that the defendant had promised to repay a debt, even though the promise was fictitious and the debt was imposed by the law. While restitutionary obligations were not enforced solely through these quasi-contractual claims, the law of quasi-contract constitutes the core of the modern law of unjust enrichment. A seminal case is Moses v Macferlan (1760) 2 Burr 1005.

...
Wikipedia