UK Prime Minister Boris Johnson has suffered a blow to his authority as MPs triggered an investigation into whether he had misled parliament and an influential former ally called on him to quit.

Mr Johnson has been fighting for political survival for months after he told parliament that his Downing Street office had followed all lockdown rules during the COVID-19 pandemic, only for an internal report to conclude it had held alcohol-fuelled parties at that time.

Police have since fined the prime minister.

He denies deliberately misleading parliament, which is a resigning matter, and says he did not realise he was breaking the rules.

He has apologised for his conduct.

During a parliamentary debate, politicians from all sides called on Mr Johnson to go and MPs backed an opposition motion that his statements “appear to amount to misleading the House” and should be investigated by its Committee of Privileges.

Mr Johnson’s Conservatives did not oppose the move and it was approved without a formal vote in the House of Commons.

The episode reignites questions about Mr Johnson’s future, with further revelations and fines over lockdown parties possible and local elections on May 5 expected to reveal that voters’ trust in his leadership has been badly damaged.

“I don’t want this thing to endlessly go on. But I have absolutely nothing, frankly, to hide,” Mr Johnson told Sky News when asked about the investigations during a visit to India.

In the parliamentary debate before MPs approved the motion, one once loyal lawmaker from the ruling Conservatives said the prime minister should now quit.

Steve Baker accused Mr Johnson of breaking the “letter and spirit” of the law, adding: “The prime minister now should be long gone… (He) should just know that the gig’s up.”

Mr Baker, a former minister, strongly supported Mr Johnson over taking the United Kingdom out of the European Union and he retains influence among some Conservative MPs after successfully co-ordinating their resistance to efforts to water down Brexit.

The approach of Mr Johnson’s government to the debate appeared in disarray, as they pulled an amendment designed to delay the motion the morning after they announced it.

The government said it was now satisfied that any parliamentary probe would only take place after the police investigation had finished, even without their amendment.

Mr Johnson’s position had recently been strengthened by his support for Ukraine following Russia’s invasion, with some of his MPs saying it is not the time to change leader.

However some Conservatives had been uneasy at the prospect of being ordered to oppose greater scrutiny of an issue that has damaged voters’ trust in the government.

Mr Johnson now could face the release of further evidence of parties at the heart of government during the stringent lockdowns he had ordered the country to observe.

He may also be fined again for further gatherings, although police said they would delay further updates on their investigation until after local elections on May 5.

To mount a challenge to Mr Johnson’s leadership, 54 Conservative parliamentarians must write letters expressing no confidence in him.

That would lead to a confidence vote and, if he lost, a contest to replace him.

Opposition leader Keir Starmer led the criticism of Mr Johnson.

“The prime minister has been accused of repeatedly, deliberately and routinely misleading this House over parties held in Downing Street during lockdown,” Mr Starmer said.

“This is a serious and grave accusation. It amounts to contempt of parliament.”