Dominic Cummings warned he is not in clear over 'chatty rat' leak

Dominic Cummings has not been cleared in the "chatty rat" leak inquiry, Britain's top civil servant is expected to make clear on Monday as Number 10 fights back against his claims.

Simon Case, the Cabinet Secretary, will be grilled by MPs about a string of allegations made by Mr Cummings, Boris Johnson's former chief adviser, in a blog post.

A Cabinet Office source said that, if asked, Mr Case will undercut two core claims made by Mr Cummings in his 1,000-word blog on Friday night.

Mr Case's office argues that the inquiry into who leaked plans for a second national lockdown in October is ongoing and nobody has been exonerated.

That counters Mr Cummings' claim that he was cleared by Mr Case of being the source – described as the "chatty rat" – who told reporters about the imminent lockdown.

"His position will be that the inquiry is still open. With the inquiry still open, it would be wrong to say that anyone is absolutely in the clear," a Cabinet Office source said.

Mr Case is also expected to say publicly that Mr Johnson has not interfered in the inquiry, despite Mr Cummings claiming that the Prime Minister had considered doing just that.

He quoted Mr Johnson as saying that because the leak inquiry could lead to an ally of his finance, Carrie Symonds, perhaps he should "stop" it.

Mr Case, 42, who became the youngest Cabinet Secretary when he took up the post last September, will appear before the public administration and constitutional affairs committee on Monday afternoon.

While the appearance is meant to be about the general work of the Cabinet Office, MPs on the committee have told The Telegraph that they expect Mr Cummings and the issue of leaks to be discussed.

On topics such as payment for Mr Johnson's Downing Street flat refurbishment, Mr Case is expected to stick as closely to the facts as possible and avoid making news.

His parliamentary committee appearance comes as the impact of the Cummings allegations continues to reverberate around Westminster.

In broadcast interviews on Sunday, Liz Truss, the International Trade Secretary, failed to deny that Mr Johnson had help from Conservative donors to cover the refurbishment of his Downing Street flat, which he has since paid back.

Allies of Ms Symonds said suggestions the Prime Minister had acted to protect her bore the traits of "1950s sexism".

Meanwhile, well-placed figures have said Mr Cummings will accuse Mr Johnson of risking lives by not closing the borders earlier in the Covid pandemic when he appears before MPs next month.

Some Conservative MPs fear the Downing Street controversies could have an impact on the local elections on May 6.

Bob Blackman, the executive secretary of the 1922 Committee of backbench Tory MPs, urged Number 10 to draw a line under the row and move on, telling The Telegraph: "We're still in the midst of a pandemic and we're coming into very important elections.

"These sorts of comments and tittle-tattle can only do damage. Let sleeping dogs lie in relation to this and get back to the important business of delivering for the people of this country."

The former Conservative leader Sir Iain Duncan Smith told The Telegraph that the row between Mr Cummings and Number 10 was a "classic Westminster story" that "comes around like bad bus services".

Sir Iain said it was a "dangerous" distraction that had caused government ministers to "start talking about it, thinking about it, worrying about it and doing stupid things because of it, which then damage them".

Appearing in front of MPs, Mr Case is expected to push back on at least two claims Mr Cummings made in his blog post on Friday, should MPs ask.

The first is that the former adviser had been cleared of being the "chatty rat" source by the inquiry, which is being run by the Cabinet Office.

Mr Cummings wrote that Mr Case told the Prime Minister that he was not the leaker, instead pointing the finger at others including Henry Newman, then an adviser to Michael Gove.

However a Cabinet Office source said: "The investigation is still live. It would be wrong to think we have landed on any one individual or, for that matter, exonerated anyone." The source added that Mr Case will make clear that no one has been exonerated if asked, which would undercut Mr Cummings' claim to have been cleared of involvement.

A senior Government source has denied Mr Newman's involvement, noting he was promoted to an adviser role to Mr Johnson earlier this year.

The second area of pushback will be on the idea that Mr Johnson had considered intervening to stop the leak inquiry because Mr Newman was a friend of Ms Symonds.

Mr Cummings quoted the Prime Minister saying Mr Newman being named the leaker would cause him "very serious problems with Carrie" and adding that "perhaps we could get the Cabinet Secretary to stop the leak inquiry".

Two well-placed Government sources told The Telegraph that Mr Case would make clear that, whatever discussions took place, the Prime Minister did not attempt to stop the inquiry.

"He will definitely say the PM did not interfere in the leak investigation," one said of Mr Case's appearance. The other said Mr Case will say there was no intervention from the Prime Minister because "it's true".

Mr Case's exact responses will depend on the questions MPs ask. Privately, committee sources have told The Telegraph that the "chatty rat" leak inquiry and allegations about the Downing Street flat refurbishment will feature.

So too will questions about Greensill Capital, the supply chain finance company and the lobbying done on its behalf by former Prime Minister David Cameron

Darren Tierney, the director general of propriety and ethics in the Cabinet Office, will be appearing alongside Mr Case.

A Number 10 spokesman said of Mr Cummings's claims: "This Government is entirely focused on fighting coronavirus, delivering vaccines and building back better."