The Whitehall investigation into the so-called “chatty rat” who leaked plans for the second coronavirus lockdown in England is still “live”, sources have said.

Boris Johnson’s former top adviser Dominic Cummings has accused the Prime Minister of seeking to shut down the investigation.

He said Mr Johnson was concerned he would face “very serious difficulties” with his fiancee, Carrie Symonds, after he was told that a close friend of hers had been implicated.

However, sources familiar with the inquiry – conducted by the Cabinet Secretary Simon Case – have indicated that it is still ongoing.

It was said that it would be “wrong” to assume that it had landed on any one individual – or had completely exonerated anyone.

The leak, on Halloween last year, was regarded as particularly serious as it was blamed for “bouncing” Mr Johnson into a new lockdown before he intended – to the fury of some Tory MPs.

In his incendiary blog post, Mr Cummings said Mr Case had told the Prime Minister that “all the evidence” pointed to Henry Newman – then a special adviser in the Cabinet Office – and others in his office.

Mr Cummings wrote: “He (Mr Johnson) said to me afterwards, ‘If Newman is confirmed as the leaker then I will have to fire him, and this will cause me very serious problems with Carrie as they’re best friends (pause) perhaps we could get the Cabinet Secretary to stop the leak inquiry?’

“I told him that this was ‘mad’ and totally unethical, that he had ordered the inquiry himself and authorised the Cabinet Secretary to use more invasive (powers) than are usually applied to leak inquiries because of the seriousness of the leak.

“I told him that he could not possibly cancel an inquiry about a leak that affected millions of people, just because it might implicate his girlfriend’s friends.”

Mr Johnson has denied the allegations saying the public did not “give a monkey’s”  about such matters.

Mr Cummings’ broadside was prompted by claims from No 10 that he had been responsible for a series of leaks against the Government – including the “chatty rat” leak – allegations he fiercely denied.

Following Mr Cummings’ departure from Downing Street amid a fierce internal power struggle with Ms Symonds, Mr Newman was promoted to become a senior adviser to the Prime Minister in No 10.

The move was seen at Westminster as sign of Ms Symonds’ increasing influence now that Mr Cummings was out of the way.

A Cabinet Office spokeswoman said: “We do not comment on leak investigations. Senior appointments at No 10 go through rigorous checks.”