Boris Johnson has been too busy with Brexit to study the report on Russian meddling in UK politics he is accused of suppressing, a minister has claimed.

Chris Pincher put forward the explanation for the refusal to publish – despite the intelligence agencies raising no objections – amid a warning it may stay secret until next summer.

The foreign office minister insisted it was the prime minister’s decision when to release the study, which was sent to Downing Street for “final confirmation” nearly three weeks ago.

And he claimed: “It is not as if the prime minister has not had one or two other things to do during the last several weeks – notably obtaining a good deal for Britain in withdrawing from the European Union.”

Unless it is published by midnight, it must remain under wraps during the campaign leading up to the general election on 12 December – despite concerning alleged interference in previous polls.

Dominic Grieve warned the delay would be far longer, until a new intelligence and security committee is put together – a process that took six months after the last election.

Tom Tugendhat, the Conservative chairman of the Commons foreign affairs committee, joined calls for the secrecy to end, so the furore “can all go away” he told the minister.

Under fierce questioning in the Commons – with several Tories calling for publication before voters go to the polls – Mr Pincher branded the controversy “a non issue”.

And he twice refused to say whether Mr Johnson had “read the report”, saying only that it “requires careful consideration” and was “going through the No 10 process”.

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The inquiry was launched after the revelation that Russia-based Twitter accounts posted more than 45,000 messages about Brexit in just 48 hours during the 2016 referendum.

Before he became prime minister, both Mr Johnson, and his predecessor Theresa May, declined to say if Russia had interfered in Brexit, arguing there was no evidence yet.

In the Commons, Mr Grieve, the now-independent former attorney general, said No 10 received the report on 17 October, adding: “It is a longstanding agreement that the prime minister will endeavour to respond within 10 days”.

“My secretariat tells me that it is unprecedented that we should have had no response at all explaining why any further delay is required in this case,” he protested.

Emily Thornberry, Labour’s shadow foreign secretary, condemned “utterly unjustifiable, unprecedented and clearly politically motivated reasons for delaying publication of this report until after the election”.

“I fear it is because they realise that this report will lead to other questions about the links between Russia and Brexit and with the current leadership of the Tory party, which risks derailing their election campaign,” she alleged.

“Publish this report and let us see for ourselves otherwise there is only one question: what have you got to hide?”

But Mr Pincher called on Ms Thornberry to “withdraw the imputation about the good name of the Conservative party and this government”.

He insisted: “There is no evidence to suggest there has been any successful Russian involvement in the British electoral cycle.”