Boris Johnson’s leadership has been plunged deeper into crisis by the shock resignation of two of his closest aides – including an ally of 14 years who quit in protest at his “scurrilous” smear linking Keir Starmer with the paedophile Jimmy Savile.

In a scathing resignation letter, policy chief Munira Mirza said there was “no fair or reasonable basis” for Monday’s attack and made clear that Johnson had resisted pleas from advisers to apologise.

Chancellor Rishi Sunak – tipped as Mr Johnson’s most likely successor if he is forced from office – distanced himself from the slur, telling reporters: “Being honest, I wouldn’t have said it.”

And Johnson’s former chief aide turned bitter enemy Dominic Cummings gleefully declared Ms Mirza’s departure “an unmistakeable signal the bunker is collapsing and this PM is finished”.

Confronted with her comments in an interview with 5News during a trip to Blackpool to publicize his “levelling up” strategy, Mr Johnson said: “I don’t agree with that.”

He again refused to apologize for suggesting that, as director of public prosecutions, Starmer was responsible for the failure to prosecute Savile for sex crimes.

Just three hours after Ms Mirza’s bombshell letter dropped, Downing Street director of communications Jack Doyle announced to No 10 staff that he too was leaving.

Mr Doyle – who had been named in the Partygate scandal for handing out awards at a 2020 Christmas drinks event in No 10 –insisted it had always been his plan to go at this point, adding that recent weeks had taken “a terrible toll” on his family life.

One Downing Street staffer told The Independent: “I think he’d wanted to leave for some time and he had been under huge pressure. He didn’t get everything right, but he tried his best for the PM.”

The insider said the atmosphere inside No 10 had been “agonising” as officials wait for the outcome of the police investigation into 12 allegedly lockdown-breaching parties as well as the publication of the full Sue Gray report.

“The atmosphere is terrible,” said the staffer. “And several people have been horrified by the Savile comment and the failure to apologise. I wouldn’t be surprised if there are more resignations soon. People are exhausted.”

The walkouts came on a fraught day for Mr Johnson’s administration, with power-sharing arrangements in Northern Ireland threatened by the resignation of DUP prime minister Paul Givan in protest over the impact of the protocol negotiated by the PM as part of his Brexit deal.

And the cost-of-living crisis facing millions of families across the UK was brought starkly home by a £693-a-year hike in the cap on domestic energy bills, forcing Mr Sunak to offer a package of assistance worth up to £350 .