It is quite possible that Mr Johnson loses his seat in the upcoming general election, while the Conservative party gains a majority.
Does the Conservative party have official procedures for what would happen?
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He would have two options if this was the case. One is he could step down and the conservative party could appoint a replacement. Second is he could technically continue in the post if he was appointed a lord.
If Mr Johnson does lose his seat, he could technically continue as Prime Minister so long as the Conservative Party win a majority. But he would have to appoint himself a Lord.
In this unlikely event, he would face an enormous amount of pressure to resign and would probably be replaced by another Tory MP in a leadership election.