Johnson acknowledged it was “clearly the will of the parliamentary Conservative party that there should be a new leader of that party, and therefore a new prime minister”.
In a six-minute speech outside 10 Downing Street that was devoid of contrition for the many missteps that brought him down, he said he would stay on until his successor is found.
But calls built for Johnson to leave immediately, and for an acting leader to head the world’s fifth-largest economy.
Conservative John Major, who was prime minister from 1990 to 1997, said Johnson’s extended tenure - and the vast powers that still go with it - was “unwise and may be unsustainable”.
The leadership election is expected to take place over the coming months. The victor will replace Johnson by the party’s annual conference in early October.
But polling suggested most Britons favour his rapid exit, amid claims that Johnson is only hanging on to enjoy a wedding party with wife Carrie at his government-funded country retreat.
Johnson’s tumultuous three years in office were defined by Brexit, the COVID pandemic and non-stop controversy about his reputation for mendacity.
The 58-year-old said he was “sad... to be giving up the best job in the world”, justifying his refusal initially to surrender to his “herd” of Tory critics because he won a personal mandate in the Brexit-dominated general election of December 2019.
Johnson also promised support for Ukraine “for as long as it takes”.
He reiterated his backing in a call afterwards to President Volodymyr Zelensky, Downing Street said.
Zelensky said he and Ukraine would be sad to see Johnson go, praising his “personal leadership” and “charisma”.
Russia expressed hope for “more professional people” to come to power in Britain.
“But at the moment there is little hope for that,” Kremlin spokesman Dmitry Peskov told reporters.
Defence minister Ben Wallace and Rishi Sunak, whose departure as finance minister Tuesday sparked the cabinet exodus, were among the early frontrunners, a YouGov survey of Tory members suggested.
Conservative MP Tom Tugendhat said late yesterday he was launching his bid to succeed Johnson, the first candidate to announce their campaign since the premier announced his resignation.
Foreign Secretary Liz Truss, another potential contender, said Johnson had “made the right decision” as she cut short a trip to Indonesia for a G20 meeting.
“We need calmness and unity now and to keep governing while a new leader is found,” she tweeted.
Even while eyeing the exit, Johnson sought to steady the ship, making several appointments to replace departed cabinet members.
They included Greg Clark, an arch “remainer” opposed to Britain’s divorce from the European Union, which Johnson had championed.
The inexperienced Shailesh Vara was put in charge of Northern Ireland, with the government locked in battle with Brussels over post-Brexit trading rules for the tense territory.
Irish premier Micheal Martin said Johnson’s exit was a chance to reset “strained and challenged” relations.
Convening the new-look cabinet after his resignation speech, Johnson confirmed his lame-duck status by saying “major fiscal decisions should be left for the next prime minister”, according to Downing Street.
As late as Wednesday night, Johnson had been defiantly clinging to power despite a wave of more than 50 government resignations.
But a fresh round of high-profile departures early yesterday, and warnings of a second no-confidence vote next week by Tory MPs, tipped the balance.
‘Arrogant and delusional’
Johnson triumphed in 2019 with a vow to “get Brexit done” following Britain’s shock referendum decision three years prior. But for many, the populist, convention-defying leader had outstayed his welcome.
The Conservative infighting erupted at a time when millions of Britons are battling the worst slump in living standards since the 1950s, fuelling by rocketing energy prices on the back of the war in Ukraine.
Johnson’s popularity had already slumped over a series of lockdown-breaking parties in Downing Street, which saw him become the first prime minister to receive a police fine.
“About time, isn’t it? Seriously, I mean have you ever known anyone be so arrogant, ignorant, delusional?” Helen Dewdney, 53, who works in consumer rights, told AFP.
While Johnson ran a successful coronavirus vaccine campaign, the former journalist also oversaw one of Europe’s worst death tolls, and nearly died himself from COVID in April 2020.
“Boris Johnson’s legacy is the deaths of nearly 200,000 British people on his watch,” said Lobby Akinnola, from the campaign group COVID-19 Bereaved Families for Justice.
“Whilst Johnson will move on to a life of writing newspaper columns and being paid eye-watering amounts to give after-dinner speeches, there will be no moving on for the families like mine that have been ripped apart by his actions,” he said.
Sunak and health secretary Sajid Javid started the ministerial exodus when they quit late Tuesday, after Johnson apologised for his February appointment of a senior Conservative MP to a prominent role in parliament.
Chris Pincher resigned as deputy chief whip last week following accusations he had drunkenly groped two men.
Downing Street officials eventually conceded that Johnson had known about other allegations against Pincher back in 2019, and many ministers recoiled at having to defend the leader yet again.
Tony Travers, director of the think tank LSE London, said the party had once again shown its propensity to turn on unpopular leaders after previously ditching premiers Margaret Thatcher and Theresa May.
“The truth is that the reason the Conservative party is so durable is that it will get rid of its leaders when it thinks they are harming the party,” he told AFP.
“And this allows the party to start again with a new leader and say, ‘look, we’re a completely different enterprise’.”