How Boris Johnson could step down as prime minister

It seems likely that Boris Johnson will resign as prime minister or be forced out within a few days. There are two options here. One is if Johnson agrees to leave No 10 when a replacement is found, as happened with Theresa May in 2019 under pressure from ministers and MPs. Under this scenario, nothing formal would need to happen, Johnson would stay in the post, attempt to patch together an interim cabinet, and the focus would shift to choosing his replacement.

If he resigned with immediate effect it would require an interim PM to be appointed, as constitutionally there needs to be a prime minister at all times. If he was standing to replace Johnson and it was felt that incumbency would give him an unfair advantage, a more neutral option is possible, but it is possible that a more neutral option could step in.

A car takes them into Buckingham Palace in a chronology known as TV news helicopter footage, where they give their resignation to the Queen, who then appoints a new prime minister on the advice of the ruling party.

This usually happens after general elections, but in recent years Tony Blair, David Cameron and Theresa May have stood down in mid-parliament. Then all of them stayed in office until a successor was chosen.

Under law, an election can be requested by the PM at any time and speaking to the liaison committee of MPs on Wednesday, Johnson hinted he could do this if colleagues try to force him out.

But to try and win an election without the blessing of the government or the bulk of your own MPs would be constitutionally very unusual, and would be fiercely resisted. A constitutional convention known as the Lascelles principle allows a monarch to refuse an election for a number of reasons, including if the parliament is still deemed viable, or if a credible alternative PM exists.

It's understood that Buckingham Palace has been taking advice on what to do if Johnson wants to win a snap election.

How will a new prime minister be chosen?

By Conservative MPs and then members, the choice is for a leader of the Tory party, who is then prime minister, as the party has a Commons majority.

The first stage is to assume that there are more than two candidates for the hopefuls to be whittled down by a series of votes by Tory MPs.

The party's backbench 1922 Committee will set out the exact method for this, but in every round of MP voting the candidate with the least support among MPs and possibly even those who fail to reach a set threshold of votes is eliminated, depending on the size of the parliamentary party at the time.

The final two are then voted on by party members, a longer process that involves a series of hustings events. In the year 2019 when Johnson replaced Theresa May, the entire leadership process took about six weeks.