One of Britain's most promising technology start-ups failed to block thousands of potentially suspicious transactions on its platform.

Documents seen by The Telegraph show that for three months last year Revolut switched off an automated system designed to stop money transfers that had been flagged for sanctions checks.

As a result, thousands of transactions passed through the London-based startup's digital banking system between July and September of 2018 without having been fully checked for sanctions compliance.

Revolut launched an internal investigation in late 2018 after a whistleblower contacted Revolut's board over serious issues with its sanctions screening system.

In a draft letter to the FCA in September 2018 (which was never in fact sent), Revolut's head of legal Tom Hambrett wrote: "The investigation concluded that the original decision to turn off the transaction-halting mechanism was erroneous and implied a remediable systems and controls failing."

Sanctions screening systems automatically flag certain payments to be subjected to further checks.

Revolut, which has grown to over 3m customers around the world since its launch in 2015, planned to contact the Financial Conduct Authority (FCA) in September to inform the watchdog of a three-month "failing", The Telegraph can reveal.

In the draft letter addressed to the FCA, Revolut wrote that it had disabled part of its sanctions screening system after it made 8,000 "false positive" flags, incorrectly identifying legitimate transactions as being potentially suspicious and blocking the transfers. The company then moved to a compliance system that flagged transactions but did not automatically disable them, Revolut said.