The Labour leader said the prime minister chose to "slander decent people" in private but lacked the "backbone to repeat it in public".

He claimed that Mr Johnson showed a different attitude "once the cameras were off" during bad-tempered exchanges at Prime Minister's Questions.

Sir Keir accused him of going "back to blaming everyone else" for his woes - including the Archbishop of Canterbury - after his partygate apology.

Politics Hub: Boris Johnson's brewing row with Church

He called it "a mealy mouthed apology when the cameras roll, a vicious attack on those who tell the truth as soon as the cameras are off. Slander decent people in a private room, let the slander spread without the backbone to repeat it in public."

His comments centred on reports that Mr Johnson had criticised the archbishop, Justin Welby, at a closed-door meeting of Tory backbenchers hours after his Commons appearance.

Sir Keir also pointed out that the prime minister's spin doctor Allegra Stratton, COVID adviser Professor Neil Ferguson and health secretary Matt Hancock had all resigned over their actions during the pandemic.

He asked: "Why does the prime minister think everybody else's actions have consequences except his own?"

Mr Johnson hit back by accusing Sir Keir of being "in some kind of Doctor Who time warp" and describing him as a "Corbynista in a smart Islington suit".

Sir Keir said the prime minister had attacked BBC coverage of the invasion of Ukraine - but Mr Johnson said this claim showed that the Labour leader was "out of his tiny mind".

It was the second day of pressure from MPs on Mr Johnson over partygate, after he apologised to the Commons on Tuesday following his fine last week for breaking lockdown rules.

Sir Keir said: "Yesterday's apology lasted for as long as the prime minister thought necessary to be clipped for the news.

"But once the cameras were off the prime minister went to see his backbenchers and he was back to blaming everyone else.

"He even said that the Archbishop of Canterbury had not been critical enough of Putin.

"In fact, the archbishop called Putin's war an act of great evil."

Mr Johnson faced fresh calls to resign from other opposition MPs including the SNP's Richard Thomson, who was told to withdraw his description of Mr Johnson as a "Pinocchio prime minister" whom the public want to see "pack his bags and go".

Plaid Cymru's Westminster leader Liz Saville Roberts asked Mr Johnson whether he will support a law banning "lying in politics".

He replied: "It is well known that the rules of this House demand that we tell the truth in this House and that's what we try to do."

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Sky's deputy political editor Sam Coates said the prime minister was trying to move on from partygate but that Sir Keir "clearly got under the prime minister's skin".

"Keir Starmer was on better form and Boris Johnson arguably slightly tetchier form than I can remember for some time," he said.

"Keir Starmer did what he doesn't always do which is to have quite specific questions which clearly got under the prime minister's skin."

On Thursday the prime minister will face a vote on a motion tabled by opposition parties calling for parliament's privileges committee to investigate whether he misled the House with his initial claims that no rules were broken during partygate - though Mr Johnson himself will be in India.

The wording of the motion - which will need to attract the support of some Tory MPs to succeed - orders that the investigation cannot begin until police inquiries into partygate have concluded.