Labour says Boris Johnson’s senior adviser Dominic Cummings faces questions over his past in Russia.
A whistleblower has apparently come forward with “serious concerns” about the relationships Mr Cummings is said to have fostered while he worked in Russia during the 1990s.
Shadow foreign secretary Emily Thornberry has written to her Tory counterpart Dominic Raab – asking if the former Vote Leave director has been given access to the government’s most sensitive “top secret” intelligence files.
It comes a day after No 10 was accused of telling “whopping untruths” over the reason for a delay to the publication of a report about the risk of Russian interference in the UK’s democratic processes.
Ms Thornberry said she did not “know the veracity” of the whistleblower’s claims but was duty-bound to pass them on in a letter also sent to the heads of MI5 and MI6 and Cabinet Secretary Sir Mark Sedwill.
“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 frontbench by a whistleblower whose motives we have no cause to question.”
The whistleblower was said to have told Labour figures to look into people Mr Cummings built up “relationships” with.
Some may have been with people involved in “politics, intelligence and security”, the individual is said to have claimed.
Dominic Cummings’ website states that he worked in Russia from 1994 to 1997 on “various projects”.
The adviser is yet to respond to Ms Thornberry’s letter, which was first reported by The Sunday Times.
The Cabinet Office said: “We do not comment on individuals’ security clearance.”
On Saturday, former attorney general Dominic Grieve, the chair of parliament’s Intelligence and Security Committee (ISC), alleged the prime minister was sitting on the dossier into the threat posed by Russia to the UK’s electoral process.
Mr Grieve says Downing Street should have signed off the report within 10 working days, but No 10 disputes this and maintains the process usually takes six weeks.
He said that Downing Street’s six-week claim on clearing the report was “completely and totally untrue… It’s a lie.”
Labour leader Jeremy Corbyn also demanded its publication, questioning what the government has “got to hide”.
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.”