Boris Johnson has lost his majority in the British parliament after one of his Conservative lawmakers made a dramatic defection in the House of Commons while the prime minister was speaking. Johnson was making an appeal to lawmakers ahead of a crucial vote this evening that could see Conservative rebels effectively block the U.K. from leaving the European Union with no trade deal. As he made his speech, Conservative Member of Parliament Philip Lee crossed the floor to join the pro-EU Liberal Democrat party. In a statement, Lee accused Johnson of “pursuing a damaging Brexit in unprincipled ways.” If Johnson loses this evening’s vote and subsequent votes Wednesday, it is widely expected he will announce a general election for next month and argue that lawmakers are ruining his negotiating position. Johnson has gambled his leadership on a “do or die” pledge to leave the EU with or without a deal by the legal deadline of Oct. 31.