British Prime Minister Boris Johnson will face a confidence vote this evening after more than 15% of elected Conservative politicians withdrew their support for him following the 'partygate' scandal.
Johnson, who helped the Conservatives win a large majority in a 2019 election, has apologised but said he would not resign after a damning official report detailed a series of illegal parties at his Downing Street office during Covid-19 lockdowns.
Below is an explanation of how the challenge to Johnson's leadership will work under the governing Conservative Party's rules.
What triggered the leadership challenge?
A leadership challenge is triggered when 15% of Conservative members of parliament (MPs) write letters demanding a confidence vote to the chairman of the party's so-called 1922 Committee, which represents party politicians who do not have government jobs.
The Conservatives have 359 MPs, so 54 letters were needed.
In a statement this morning, 1922 Chairman Graham Brady said that the threshold had been exceeded and a vote of confidence was scheduled for 6pm to 8pm this evening.
What will happen during the confidence vote?
All 359 Conservative MPs can vote, so if Johnson gets 180 votes, it will secure the majority he needs to survive.
There is little formal indication of the extent of the rebellion against Johnson.
Not all of those who submitted letters did so publicly, while around two dozen called for him to go or criticised him without confirming they sent a letter.
Senior members of Johnson's cabinet have expressed their support for him.
The Institute for Government estimates that around 160-170 politicians hold government roles in some capacity, and would usually be expected to resign if they wished to oppose the government in an issue of importance.
While some have already quit government over 'partygate', the confidence vote is run by Conservative politicians and the ballot is a secret one.
If Johnson wins, he remains in office and cannot be challenged again for 12 months.
If he loses, he must resign and is barred from standing in the leadership election that follows.
What happens if Johnson wins the confidence vote?
Although Johnson would theoretically be safe in his job for a year, that rule could be changed.
Asked whether that timeframe could be shortened, Brady said: "Technically, it's possible for rules to be changed, but the rule at present is that there will be a period of grace."
Even if Johnson is protected from a new vote of confidence, a significant mutiny could make his leadership untenable.
Johnson's predecessor Theresa May won a confidence vote in her leadership in December 2018, by 200 votes to 117. However, the pressure on her to quit continued after she failed to deliver Britain's departure from the European Union on time and she announced her resignation less than six months later.
What would happen if Johnson loses?
If Johnson loses, there will be a leadership contest and his replacement would become prime minister.
If several candidates come forward, a secret vote is held among Conservative MPs to whittle down the field.
The candidate with the fewest votes is removed and another ballot among Conservative politicians is held.
The process is repeated until two candidates remain, with votes typically held several days apart on Tuesdays and Thursdays.
The final two candidates are then put to a postal ballot of the wider Conservative Party membership, with the winner named the new leader. Voters need to have been party members for more than three months.
The length of time the leadership contest takes can vary, depending how many people put themselves forward.
May became leader less than three weeks after predecessor David Cameron resigned after all other contenders dropped out.
Johnson faced former health minister Jeremy Hunt in the run-off ballot of Conservative members to replace May, and took office two months after May announced her intention to resign.