The chance of a vote on the prime minister's leadership grows as more Conservative MPs go public with their letters of no confidence in Boris Johnson. This is how the process works:
The rules stipulate that 15% of Conservative MPs must submit a letter of no confidence to Sir Graham Brady, chair of the backbench 1922 Committee, in order for a ballot to be held.
The MPs don't have to reveal their identity, though some will choose to go public due to pressure from their constituents or in a bid to encourage others to do the same.
There are 54 MPs who need to force a vote of no confidence, and they can either deposit a letter personally to Brady's office, ask a colleague to drop it off for him or email it directly.
Brady is the only person who knows how many letters have been submitted. Several rumours have been spread to discourage people from submitting no confidence letters because of the level of mistrust among Conservative MPs.
Brady does not need to ring every Conservative MP who has submitted a letter to make sure they are happy with their decision. Nor does he, along with others, suggest passing a list of those who have submitted letters to Johnson's ministerial aides, known as parliamentary private secretaries.
There have been reports that government whips have previously been posted to keep watch out of Brady's office to spy on those going in and out.
What happens if the 54 threshold is hit?
There is a certain amount of discretion afforded to Brady, but he is expected to tell the prime minister and then organise a private confidence vote as soon as practicable. Most MPs agree that it is not practical to hold a ballot during the Queen's Platinum Jubilee bank holiday weekend.
In December 2018, Brady told Theresa May that the threshold had been hit and a vote was held on her premiership the following day. The procedure was seen as a bid to give as little time as possible to her opponents to organise against the current government.
A ballot box will be dusted off in the office of one of the executive members in the year 1922 if a no confidence vote is called.
It will probably be placed in the same room where MPs gather to meet every week for committee meetings, on the first floor of the Palace of Westminster and in the centre of a long corridor overlooking the River Thames.
All Conservative MPs will be able to vote in a secret ballot throughout the day. They are not allowed to take pictures inside the committee room. Even though they are on the government payroll, they are able to decide whether to vote for or against Johnson, as a result of the fact that party whips have been unable to show evidence that an MP has voted for or against Johnson.
It is not to say that supporters and opponents of the prime minister will not spend the day lobbying and counting the pledges of each MP collared to ask how they voted.
If an MP is away from Westminster, they can nominate a colleague to vote on their behalf as a proxy.
How does the result look like?
Brady will probably do as he did in 2018, and assemble MPs and journalists in the same parliamentary committee room where voting took place that day. He will then declare that the parliamentary party does not have confidence in the prime minister, and reveal the number of votes cast in both directions.
If Johnson wins, no more confidence votes can be held by the Committee for another year.