The decision to launch a fresh investigation – the third into the “partygate” allegations following inquiries by the Met Police and senior civil servant Sue Gray – begins a torrid few weeks for Mr Johnson.

He is expected to face substantial losses in the local elections on May 5, more fines from the Met for his own alleged rule-breaking and the publication of Ms Gray’s full report.

A by-election in the Red Wall seat of Wakefield is also imminent following the resignation of Imran Ahmad Khan, its Tory MP, after he was convicted of child sex offences.

Mr Johnson will also have to lay out his political agenda at the State Opening of Parliament on May 10, amid concern that domestic priorities have been sidelined.

Thursday’s decision in the Commons means MPs have formally ruled that Mr Johnson appeared to mislead Parliament when he said in December that no Covid rules had been broken in Downing Street. The privileges committee must now decide whether he knew that not to be true.

It has the power to request additional evidence from Ms Gray or the Met to assist its investigation, including a dossier of photographs of Downing Street parties that would otherwise remain secret. The committee could choose to publish anything it receives.

As well as ruling on whether Mr Johnson intentionally misled the Commons – which would be likely to trigger more letters of no confidence from his own MPs and calls for his resignation – the committee can recommend its own sanctions.

That could see Mr Johnson suspended from the Commons for a fixed period. A suspension of more than 10 days could in turn prompt a recall petition and a by-election.

The committee is made up of seven MPs – four Tories, one SNP and two Labour – but Chris Bryant, one of the Labour members, has recused himself over his previous criticism of the Prime Minister.

On Thursday, several MPs who had previously supported the Prime Minister over “partygate” declared their support for the investigation and called for him to resign.

Mr Baker, a senior backbencher who backed Mr Johnson to stay in Downing Street on Tuesday, said he had changed his mind because his apology for receiving a fixed penalty notice had “only lasted as long as it took to get out of the headmaster’s study”.

“The Prime Minister should just know the gig’s up,” he said.