The shadow foreign secretary, Emily Thornberry, has written to the government with questions about Dominic Cummings’s connections to Russia and the levels of security vetting to which he has been subjected in Downing Street after an official-level whistleblower raised serious concerns.

Cummings, the divisive senior adviser to the prime minister, Boris Johnson, spent three years in Russia from 1994 to 1997 after he graduated from Oxford University with a first in ancient and modern history.

In her letter addressed to the foreign secretary, Dominic Raab, which was first reported on in the Sunday Times and has been seen by the Guardian, Thornberry says shadow ministers have been approached with “serious concerns by an official-level whistleblower” but adds: “We make no claim to know the veracity of their claims.”

She then asks what level of security vetting Cummings was subjected to prior to his appointment at No 10.

“I would assume that – given the seniority of his position and the influence it gives him over decision-making at the top of government – that he was subject to the highest level of developed vetting (DV) and that – as a result – he is able to study ‘top secret’ intelligence material and attend meetings on the UK’s military and security operations overseas,” she writes.

The Cabinet Office said: “We do not comment on individuals’ security clearance.” The Guardian approached Downing Street, which had nothing further to add to the Cabinet Office statement.

The shadow foreign secretary asks a number of further questions in relation to Cummings’s relationship with Russia, on the assumption that he has been subjected to the highest level of developed vetting. She asks:

What was Cummings’s relationship with academics at Oxford University, and did they discuss the possibility of working to support the post-communist Russian government?

Was Cummings questioned about the purpose of his three-year period of work in post-communist Russia between 1994 and 1997, including relationships with figures in Russian politics, intelligence and security?

Was Cummings questioned about relationships with members of the group Conservative Friends of Russia?

The letter is copied to Sir Mark Sedwill, head of the civil service, as well as the heads of MI5 and MI6 and the chair of the intelligence and security committee (ISC), Dominic Grieve.

Grieve this week accused the prime minister of sitting on an ISC report on Russian interference in British politics, including the 2016 EU referendum. The ISC said it had expected Johnson to approve publication of the 50-page dossier by Thursday – and there was now a risk its publication would be prevented before the general election.

It is understood that the dossier examines allegations that Russian money has flowed into British politics in general and the Conservative party in particular. It also features claims that Russia launched a major influence operation in 2016 in support of Brexit.