A sudden change of government
MPs) is reached, a vote of Conservative MPs is triggered. If she loses, Mrs May is barred from standing in the election that then takes place for a new leader.
Could there be a general election? Before the Fixed-term Parliaments Act 2011, a PM could make a major vote an issue of confidence, meaning that if the vote was lost then either the Government would resign or there would be a general election. Under the Act, the PM can no longer unilaterally call an election. It requires either a two-third vote of all MPs that there be an early election, or a vote of no confidence being carried (and within 14 days no new government being formed and winning a confidence vote).
The PM could say that, if she lost on Tuesday, she would ask the House for an early election but would be dependent on the votes of the very MPs who have rebelled and caused her to take that step.
One other scenario is for her to say that, if defeated on the agreement, the Government will resign. That would not engage any provision of the Act, so creating major political uncertainty as to what would happen next. The alternative would be the Opposition moving a vote of no confidence. Much would then depend on the votes of the DUP MPs.
In short, there could be a situation of political and constitutional turmoil, all while the clock ticks. However MPs vote this week, the UK is leaving the EU on March 29 next year.
Philip Norton (Lord Norton of Louth) is Professor of Government at the University of Hull.