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The source who leaked Sir Kim Darroch’s diplomatic cables was exacting revenge over the ambassador's alleged failure to promote a pro-Brexit Britain in Washington, it was being claimed last night.

Mystery surrounds who leaked the ‘bombshell’ memos and why, prompting the Foreign Office to launch a formal inquiry. But it is understood the source of the leak - either a senior minister or official - did so to highlight their frustration at the failure of the civil service to push Brexit.

According to a source close to Isabel Oakeshott, the political journalist who obtained the leaked memos, they were passed to her - and then published in the Mail on Sunday - because she is a well-known Brexiteer.

The source said: “There are pro-Brexit officials who feel like pariahs and believe the Civil Service is institutionally pro-Remain.

“There is certainly a view that Sir Kim with his long background in Brussels and as a former adviser to Tony Blair on Europe had not made the most of his time in Washington to explore post-Brexit opportunities.”