Debate in the House of Commons
MPs will finally get their chance to say yes or no to May's deal after more than two years of political upheaval, with all signs indicating a resounding rejection.
As Tuesday's debate kicked off, House Speaker John Bercow announced which amendments he had chosen from more than a dozen submissions.
They indicate the huge range of opinion among lawmakers about how Britain should pursue Brexit. One amendment from the opposition Labour Party rejects May's deal and also rules out leaving the bloc without agreement, while one from the Scottish National Party seeks to postpone Britain's departure from the EU.
The other two amendments seek to limit the most contentious section of the deal, a temporary customs union with the EU intended to keep the border between Northern Ireland and Ireland barrier-free after Brexit.
A "no" vote on Tuesday evening would throw British politics into further turmoil, just 10 weeks before the country is due to leave the EU on 29 March.
It is not clear if it would push the government toward an abrupt "no-deal" break with the EU bloc or nudge it toward a much softer departure, or even a second referendum to test over sentiment.
The vote is also a test of May's authority. She heads a minority government - making her vulnerable - and has made delivering Brexit her main focus since taking office in 2016.
Voting is due to begin around 1900 GMT.