Русско-англо-русский юридический словарь

letter before action. a formal notice demanding that the recipient perform a legal obligation to avoid being taken to court, eg pay a sum of money or perform an obligation under a contract (TransLegal)

A letter before claim (sometimes known as a ‘letter before action’) is a letter putting a person on notice that court proceedings may be brought against them. These days the courts take the view that litigation should be a last resort and, therefore, parties are encouraged to resolve their disputes at an early stage by communicating with each other, providing information and documentation to each other and considering alternative dispute resolution (ADR) procedures. (In Brief)

Before court proceedings are commenced, a claimant should consider sending a Letter of Claim to the defendant. (lexlaw.co.uk)

A letter before action (also known as a 'letter of claim' or 'letter before claim') is the first step in a formal debt recovery process. (girlings.com)