Saturday’s vote in the 650-seat House of Commons is expected to be knife-edge close and comes less than two weeks before the UK is scheduled to depart the bloc on October 31.
If he wins, Johnson will have delivered Brexit, as he has repeatedly pledged to do since taking office in July. If he loses, however, the UK’s years-long political drama over its decision to quit the EU in a 2016 referendum will once again kick into overdrive.
Here is what you need to know.
When will parliament vote?
MPs will sit from 08:30 GMT (9:30am local time) in parliament’s first Saturday session this century. MPs last convened on a Saturday in April 1982, when they met to discuss the Argentine invasion of the British-controlled Falkland Islands.
Johnson will make a statement to MPs and will then take questions on his deal. That will be followed by a debate, which is not time-limited and could last several hours. The vote will take place after these discussions have concluded, meaning it could start anytime from the early afternoon onwards.
How will MPs decide?
Johnson’s statement is his last chance to convince MPs to back his plan.
At the end of their debate, the vote will be decided by a simple majority and the result announced soon after.
Johnson requires at least 320 votes to guarantee victory, if there is full attendance. The number is less than 326 because seven Irish nationalist Sinn Fein party members do not take their seats and the House of Commons’ Speaker and the Speaker’s three deputies do not vote.