Mr Burns said the PM was determined to “rebuild trust” with the public.

But a senior Tory MP has said it is “a matter of when not if” the PM faces a no confidence vote from his own MPs.

Mr Johnson is also facing a Commons investigation into whether he deliberately misled MPs by telling them Covid laws in No 10 were followed.

No 10 had tried to delay the vote on launching the inquiry, but U-turned at the last minute meaning the motion passed unopposed.

Last week, Mr Johnson, his wife Carrie and the Chancellor Rishi Sunak were fined by the police for breaching lockdown rules by attending a gathering to celebrate his birthday.

The police have confirmed they have issued at least 50 fines so far.

Speaking to the BBC’s Today programme, chair of the Defence Committee Tobias Ellwood said Conservative MPs were “deeply troubled” and warned that the ongoing row about lockdown parties was doing “long-term damage to the party’s brand”.

He accused No 10 of lacking “discipline, focus and leadership” and predicted the prime minister would face a vote of no confidence, which could force him out of office.

The Bournemouth East MP also dismissed suggestions Mr Johnson should stay in post to manage the Ukraine crisis accusing the prime minister’s allies of using the war as a “fig leaf”.

BBC political correspondent Jonathan Blake said that one MP, a critic of the PM, said the government’s U-turn and fallout was “predictable”, while a loyalist to Boris Johnson admitted the Partygate saga had “polarised” him and his colleagues.

A critical MP pointed to Conservative Steve Baker’s contribution in the Commons debate on Thursday, when Mr Baker removed his support from Mr Johnson.

Mr Baker and another critic, former chief whip Mark Harper, are influential voices in the party and the MP pointed out that they are notorious organisers, although there is no suggestion there is an immediate move to unseat the prime minister.

Speaking during a debate in the House of Commons Mr Baker said he had been “deeply moved” when the prime minister apologised to MPs for breaking the rules.

However, he said the contrition “had only lasted as long as it took to get out of the headmaster’s study” and that the PM should realise “the gig is up”.

Earlier in the week, Mr Harper said the prime minister was “not worthy” of the office and should step down.

Mr Burns dismissed criticism from Conservative MPs saying some of his colleagues had never supported the prime minister.

“If the prime minister stepped off Westminster Bridge and walked down the Thames on top of water they would say it was because he couldn’t swim,” he said.

He said Mr Johnson still “had work to do” and cited the cost of living crisis and continued complications over Brexit arrangements in Northern Ireland.

Labour shadow minister Peter Kyle said Mr Johnson was a “dead man walking” who could no longer manage the country or his own party.

Liberal Democrat deputy leader Daisy Cooper said the partygate issue was “a huge distraction for Boris Johnson and the government, and this is why he should have resigned a long time ago.”

Mr Johnson is currently on his final day of an official trip in India, where he met Indian Prime Minister Narendra Modi.

The two men agreed they wanted to secure a free trade deal between the two countries, with Mr Johnson saying it should be done by October.

Following a debate in the House of Commons, MPs have asked the Privileges Committee to investigate whether Mr Johnson deliberately misled Parliament.

If the committee finds Mr Johnson did mislead Parliament, they can recommend a sanction which could include a suspension or expulsion from Parliament entirely. They could also recommend he apologises to the House.

MPs will then decide whether or not to approve the report and implement the recommended sanctions.

The Privileges Committee is made up of seven MPs – two Labour MPs (Yvonne Fovargue and Chris Bryant), one SNP (Allan Dorans) and four Conservatives (Andy Carter, Alberto Costa, Laura Farris and Sir Bernard Jenkin).

However, the committee chair Mr Bryant has recused himself from the investigation as he had already commented publicly on the matter.