Johnson has weathered several storms during his time as prime minister, but this could be one crisis too many.
Here’s what you need to know.
The immediate cause of the crisis was the fallout from Deputy Chief Whip Chris Pincher’s resignation last Thursday amid allegations that he groped two guests at a private dinner the night before.
What got Johnson into more trouble, however, was the contortions Downing Street press secretaries went into trying to explain why Pincher was ever in government.
When new reports of Pincher’s historic behavior emerged in the light of his resignation, Downing Street initially denied that the Prime Minister knew anything about the allegations relating to Pincher’s time as Foreign Secretary.
When it became clear that would not apply, Johnson’s team said they were aware of the historic allegations but that they had been “resolved”.
When it emerged that one of the previously unreported allegations against Pincher had been confirmed, Johnson’s spokesman explained that “resolved” could mean it had been confirmed.
On Tuesday morning, Simon McDonald, the former top official at the Foreign Office, revealed that Johnson had been personally briefed on the outcome of an investigation into Pincher’s conduct, sparking a spate of resignations throughout the day.
What happens next?
Boris Johnson is still in control of his own destiny… for now.
Conservative Party rules dictate that a leader who wins a vote of confidence is immune from a further challenge for twelve months. Johnson survived a June 6 vote of confidence.
But the current crisis is so profound that it’s possible that the 1922 Conservative backbench committee could rewrite the rules to get rid of the Prime Minister.
The committee met on Wednesday and decided to hold an election for a new leadership on Monday. Once elected, the committee’s new head will decide whether to change the rules and proceed with another vote of confidence – one that Johnson would be far more likely to lose.
Until then, the question is how much public humiliation Johnson can take. Dozens of MPs have now left government and a delegation of cabinet members descended on Downing Street on Wednesday night to call on the Prime Minister to resign.
One of them – Britain’s Home Secretary Priti Patel – told Johnson that the party’s general view was that he had to go, a source close to Patel told CNN.
More government ministers will almost certainly resign, and opposition circles are talking of resignations.
What happens if Johnson resigns?
In the UK, the resignation of a prime minister does not automatically trigger snap elections.
Should Johnson resign, the Conservative Party would hold an internal election to choose a new leader, who would then become Prime Minister.
Johnson would likely remain in office until his successor was elected, as would his predecessors Theresa May and David Cameron when they resigned in May 2019 and June 2016, respectively.
Subject to another resignation or a snap election, the new Prime Minister would then lead the UK until the next scheduled general election in 2024.