Johnson announced his resignation on Thursday, a dizzying about-face after months spent insisting he would stay in his job amid mounting ethics scandals and growing Conservative discontent.
He quit as party leader with a statement to the nation outside 10 Downing St. but said he would stay in his post as prime minister until his successor is chosen by the party.
That decision didn't sit well with some of his Conservative colleagues, who worry Johnson lacks the authority to hang on or could do mischief even as a caretaker prime minister.
James Cleverly, appointed as education secretary on Thursday after his predecessor quit during a mass exodus of ministers, defended Johnson's decision to stay.
``It's right that he has stood down and it's right that he has put a team in place to continue governing whilst the selection procedure flows for his successor,'' Cleverly told Sky News. ``And we should do that I think pretty quickly, pretty promptly.''
Party officials are due to set out the timetable for a leadership contest on Monday, with the aim of having a winner by the end of the summer. The two-step process involves Tory lawmakers voting to reduce the field of candidates to two, who will go to a ballot of all party members across the country.
Lawmaker Tom Tugendhat, who chairs the House of Commons' influential Foreign Affairs Committee, became the second candidate to declare he is running, after Attorney General Suella Braverman. Former Health Secretary Sajid Javid and ex-Treasury chief Rishi Sunak _ whose resignations this week helped topple Johnson, are also likely contenders, along with Foreign Secretary Liz Truss, Defense Secretary Ben Wallace and Transport Secretary Grant Shapps.
Even as contenders launch their campaigns, Johnson remains in office atop a caretaker administration formed from a dwindling band of loyalists alongside ministers who have agreed to stay in office to keep the government running.
Johnson has promised not to make any major policy decisions in his remaining time, but many Conservatives say a lame-duck leader is the last thing the country needs amid Russia's war in Ukraine and a worsening cost-of-living crisis triggered by soaring food and energy prices.
Some also are wary of Johnson's intentions after a resignation speech in which he made clear he didn't want to leave, but had failed ``to persuade my colleagues that it would be eccentric to change governments when we're delivering so much and when we have such a vast mandate.''
George Freeman, who quit as science minister on Thursday, said he worried about a leadership election being held in ``a febrile moment of midsummer madness, where we choose the wrong person in a hurry because of the instability.''
Some had pushed for Johnson to give way and let Deputy Prime Minister Dominic Raab step in as temporary leader. But lawmaker Geoffrey Clifton-Brown, treasurer of the Conservative committee that runs party leadership contests, said ``that ship has sailed.''
``We must now live with the fact that Boris Johnson will be prime minister until a successor can be voted on,'' he said.
The main opposition Labour Party said that was unacceptable and vowed to call for a no-confidence vote in Johnson in the House of Commons next week, though prospects of its success were uncertain.
``He's a proven liar who's engulfed in sleaze and we can't have another couple of months of this, you know,'' Labour deputy leader Angela Rayner said. ``So they do have to get rid of him, and if they don't, we will call a no-confidence vote because it's pretty clear, he hasn't got the confidence of the House or the British public.''
The brash, 58-year-old politician who took Britain out of the European Union and has been at the helm at the height of COVID-19 and the war in Ukraine survived scandals over his expenses and lockdown-breaking government parties during the pandemic. He was brought down by one scandal too many, this one involving his appointment of a politician who had been accused of sexual misconduct.
Johnson faced days of questions, and gave days of conflicting answers, over what he knew about past allegations against Chris Pincher, a Conservative lawmaker who resigned as party deputy chief whip last week after allegedly groping two men at a private club. Pincher acknowledged he had gotten drunk and ``embarrassed myself.''
Johnson offered to shift explanations about what he knew and when he knew it. That just brought concerns the prime minister couldn't be trusted to boiling point.
Javid and Sunak, key Cabinet members who were responsible, respectively, for fighting COVID-19 and inflation, resigned within minutes of each other Tuesday, setting off a wave of departures by their colleagues.
Johnson clung to power for days, defiantly telling lawmakers on Wednesday that he had a ``colossal mandate'' from the voters and intended to get on with the business of governing.
His resignation the next day was a humiliating defeat for a politician whose jokey bluster brought a celebrity status unmatched in British politics, but who was accused of behaving as if the rules didn't apply to him.