Boris Johnson's senior adviser Dominic Cummings is facing questions from Labour over his past in Russia after an apparent whistleblower came forward with "serious concerns".
Shadow foreign secretary Emily Thornberry has written to Tory counterpart Dominic Raab questioning whether the former Vote Leave director has been granted access to the Government's most sensitive "top secret" intelligence files.
In a letter copied to the heads of MI5 and MI6 and Cabinet Secretary Sir Mark Sedwill on Friday, she said Labour did not "know the veracity of their claims" but felt compelled to inform them.
She wrote: "I am writing to you based on the serious concerns that have been raised with shadow ministers by an official-level whistleblower," she wrote.
"I feel duty-bound to put to you the concerns raised with the Labour front bench by a whistleblower whose motives we have no cause to question."
Major questions over Cumming's past
The individual was said to have approached Labour figures to question "relationships" Mr Cummings may have fostered with people involved in "politics, intelligence and security" in Russia when he worked there in the 1990s.
Mr Cummings is yet to respond to a request for comment on the letter first reported by the Sunday Times.
The Cabinet Office said: "We do not comment on individuals' security clearance."
The questions came as the Prime Minister faces calls to publish a key report assessing the threat posed by Russia to Britain's democratic processes.
Former attorney general Dominic Grieve, who has seen the dossier, stressed its publication is essential ahead of the General Election, with it containing information "germane" to voters.
Liberal Democrat leader Jo Swinson also called on the Government to stop "keeping it a secret".
Mr Grieve, chairman of the Intelligence and Security Committee at Parliament, has accused the PM of sitting on the report ahead of the December 12 vote.
The independent MP, who was exiled from the Tories by the PM over his no-deal Brexit opposition, is calling on the Conservative leader to publish the committee's report before Parliament is dissolved on Tuesday.
A Downing Street spokeswoman said: "There are a number of administrative stages/processes which reports such as this – which often contain sensitive information – have to go through before they are published.
"This usually takes several weeks to complete. The committee is well informed of this process."