The prime minister is determined to fight for his political survival and vowed to "keep going" during a fierce session of PMQs on Wednesday - but history suggests that time is running out on his premiership.
So, where does the prime minister go from here and is there a realistic possibility he will be able to retain his premiership?
What happened on Tuesday?
Mr Sunak and Mr Javid stepped down within minutes of each other on Tuesday evening in protest over the PM's leadership.
It came just hours after Downing Street confirmed Mr Johnson was informed that former deputy chief whip Chris Pincher had been investigated over his conduct when he was a Foreign Office minister - following days of denials from Downing Street that the PM knew.
Now ex-chancellor Mr Sunak said in his resignation letter that “the public rightly expect government to be conducted properly, competently and seriously” and “I believe these standards are worth fighting for and that is why I am resigning”.
He also hinted at splits on economic policy, pointing to the need to “work hard, make sacrifices and take difficult decisions” – a planned joint speech with the prime minister had made it clear “our approaches are fundamentally too different”.
Mr Javid said the public had concluded that under Mr Johnson the Tories were not “competent in acting in the national interest” and the prime minister could not offer “humility, grip and new direction”.
Their letters triggered a series of resignations from other ministers on Tuesday evening and more followed on Wednesday morning.
Why are these resignations so significant? Robert Peston explained on Tuesday's News at Ten
Why have they resigned now?
The prime minister’s judgment has once again been called into question over his handling of the Chris Pincher row.
The former deputy chief whip quit last week after he “drank far too much” and “embarrassed myself” at the exclusive Carlton Club, where he allegedly "groped" two male guests.
The latest disclosure comes after details emerged in the press over the weekend of further claims about alleged sexual advances to men – including two fellow Conservative MPs – over a period of years.
No 10 had previously claimed the prime minister was not aware of any "specific allegations" against Mr Pincher when he appointed him as deputy chief whip - a key ministerial role.
But Mr Johnson was forced into a humiliating apology after admitting he knew about previous inappropriate behaviour by Mr Pincher when he was a Foreign Office minister in 2019 but still made him deputy chief whip.
Asked if it was an error to give Mr Pincher a role in his government, he said: “I think it was a mistake and I apologise for it.”
Are there any other reasons behind their resignations?
Plenty. Just last month saw Mr Johnson scrape through a no confidence vote with 41% of Tory MPs saying they had no confidence in the prime minister. Among their concerns were his personal style, economic policy and the Sue Gray report into lockdown-breaking parties in Downing Street motivating the rebels.
Some Tory MPs were prepared to back Mr Johnson, who led the party to a landslide electoral win in 2019, because of his popularity with voters.
But the devastating by-election defeats in the key areas of Tiverton and Honiton and Wakefield, which led to the resignation of Tory chair Oliver Dowden, cast doubt on the idea of Mr Johnson remaining an electoral asset to the party.
What next for Mr Sunak and Mr Javid?
They both have leadership ambitions and are in a position to inflict further wounds on the prime minister.
As resigning ministers they had the opportunity to make statements in the Commons explaining their reasons.
Mr Javid launched a stinging attack on Mr Johnson during PMQs telling him "enough is enough" and that being one of his ministers meant he was having to chose between "loyalty and integrity" and said he "will never risk losing my integrity".
Those with long memories in Westminster recall Geoffrey Howe’s devastating resignation statement in 1990 which helped topple Margaret Thatcher.
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Is it just the usual suspects that the prime minister needs to worry about?
In short no. As well as a host of resignations from junior ministers, a number of MPs have also submitted letters withdrawing their support for the PM, including Lee Anderson and Tom Hunt who had been fervent supporters of Mr Johnson.
"I cannot look myself in the mirror and accept this", Mr Anderson said, adding: "Integrity should always come first and sadly this has not been the case over the past few days."
Former education minister Will Quince said he felt he had "no choice" but to quit after being sent to broadcast interviews on Monday to defend the PM over the Chris Pincher row with information from Number 10 which has "now been found to be inaccurate".
Jonathan Gullis, MP for red wall seat Stoke-on-Trent North and previously a Johnson loyalist, resigned as a ministerial aide.
“I feel for too long we have been more focused on dealing with our reputational damage rather than delivering for the people of this country and spreading opportunity for all, which is why I came into politics,” he said.
But the prime minister could continue to count on staunch allies Conor Burns and Nadine Dorries who both gave public messages of support.
So is this the end?
It’s only days since Mr Johnson said he was looking ahead to a third term in office which would keep him in No 10 into the 2030s, so resigning does not appear to be on his mind.
Asked by a Tory MP whether there was any circumstance in which he would quit, the PM told the Commons on Wednesday: "The job of a prime minister in difficult circumstances when he has been handed a colossal mandate is to keep going and that's what I'm going to do."
Mr Johnson may have survived a no confidence vote, but so did his predecessor Theresa May, who was forced to resign within six months as her backbench support collapsed.
He's immune for a year from another vote on his leadership, but there is talk among backbenchers about changing the rules to allow another attempt to oust him.
For that to happen, the rules of the 1922 Committee of backbenchers will need to be changed to allow another confidence vote within 12 months.
Unhappily for the prime minister, vacancies on the committee’s executive could be filled by critics who are ready to make that rule change.
Allies of Mr Johnson warned that would leave any successor as Tory leader with a “gun to their head” and the prospect of a confidence vote at any time if the threshold of 15% of MPs calling for one is met.
Can he survive?
Under normal political rules, a prime minister in Mr Johnson’s position would probably already be calling the removal vans to Downing Street.
But Mr Johnson has made a career out of defying political gravity and still has a comfortable Commons majority.
David Cameron suggested his fellow Old Etonian Mr Johnson is a “greased piglet” and the prime minister may yet find a way to save his bacon.