The leak has caused outrage among MPs, who circulated theories in parliament that the culprit could be Russian government hackers or another hostile state.
Isabel Oakeshott, the journalist who received the leak, is close to a number of pro-Brexit politicians and co-wrote an account of the EU referendum with Arron Banks, the leave-supporting donor, who is under investigation by the National Crime Agency over the sources of funding for the Leave.EU campaign.
However, Alan Duncan, a Foreign Office minister, told the Commons it was the government’s belief that the leak made its way to Oakeshott “from within” Whitehall, rather than being handed over by foreign agents. He said the Cabinet Office had begun its formal hunt for the mole among current senior politicians, aides and civil servants, but did not at this stage think it was right to involve the police.
He said one of the documents was saved from 2017, but three of the memos were produced just eight to 10 days ago, which appeared to point to someone who was still in post.
The cross-government inquiry has already begun examining who had access to the recent memos – which went to potentially hundreds of people – and a highly sensitive letter from 2017 that had a much narrower circulation list.