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Scandal-ridden Boris Johnson has announced he will quit as British prime minister after he lost the support of his ministers and most Conservative lawmakers, but says he will stay on until his successor is chosen.

Bowing to the inevitable as more than 50 ministers quit and lawmakers said he must go, an isolated and powerless Johnson said it was clear his party wanted someone else in charge.

"Today I have appointed a cabinet to serve, as I will, until a new leader is in place," Johnson said outside his Downing Street office where his speech was watched by close allies and his wife Carrie.

"I know that there will be many people who are relieved and perhaps quite a few who will also be disappointed. And I want you to know how sad I am to be giving up the best job in the world. But them's the breaks."

Johnson gave no apology for the events leading to his announcement and said his forced departure was "eccentric".

There were cheers and applause as he began his speech, while boos rang out from some outside the gates of Downing Street.

The Conservatives will now have to elect a new leader, a process which could take weeks or months.

Many said Johnson should leave immediately and hand over to his deputy, Dominic Raab, saying he had lost the trust of his party.

Keir Starmer, leader of the main opposition Labour Party, said he would call a parliamentary confidence vote if the Conservatives did not remove Johnson at once.

"If they don't get rid of him, then Labour will step up in the national interest and bring a vote of no confidence because we can't go on with this prime minister clinging on for months and months to come," he said.

Support for Johnson had evaporated during one of the most turbulent 24 hours in recent British political history, epitomised by finance minister, Nadhim Zahawi, who was only appointed to his post on Tuesday, calling on his boss to resign.

Zahawi and other cabinet ministers had gone to Downing Street on Wednesday evening, along with a senior representative of those lawmakers not in government, to tell Johnson the game was up.

Initially, Johnson refused to go and seemed set to dig in, sacking Michael Gove - a member of his top ministerial team who was one of the first to tell him he needed to resign - in a bid to reassert his authority.

But by Thursday morning as a slew of resignations poured in, it became clear his position was untenable.

There had been so many ministerial resignations that the government had been facing paralysis.

The ebullient Johnson came to power nearly three years ago, promising to deliver Brexit and rescue it from the bitter wrangling that followed the 2016 referendum.

Since then, some Conservatives had enthusiastically backed the former London mayor while others, despite reservations, supported him because he was able to appeal to parts of the electorate that usually rejected their party.

That was borne out in the December 2019 election. But his administration's combative and often chaotic approach to governing and a series of scandals exhausted the goodwill of many of his lawmakers while opinion polls show he is no longer popular with the public at large.

The recent crisis erupted after lawmaker Chris Pincher, who held a government role involved in pastoral care, was forced to quit over accusations he groped men in a private member's club.

Johnson had to apologise after it emerged that he was briefed that Pincher had been the subject of previous sexual misconduct complaints before he appointed him. The prime minister said he had forgotten.

This followed months of scandals and missteps, including a damning report into boozy parties at his Downing Street residence and office that broke COVID-19 lockdown rules and saw him fined by police over a gathering for his 56th birthday.

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