Boris Johnson was dealt twin blows on Tuesday evening when a dramatic resignation cost him his razor-thin majority in Parliament and rebel lawmakers cleared the first hurdle in their plot to block to the Prime Minister's Brexit plans.

Members of Parliament opposed to Britain crashing out of the European Union without a deal on October 31 — as Johnson has threatened to do — called Tuesday for an emergency debate that would allow them to seize control of the agenda from the government in the House of Commons tomorrow.

The House of Commons Speaker John Bercow, allowed the debate to go ahead, paving the way for a vote later on Tuesday. If it passes, rebel MPs — including a number of lawmakers from Johnson's own Conservative Party — will take control of the business in the House on Wednesday and could pass legislation that would delay Brexit and outlaw the no-deal scenario.

Johnson is expected to seek an early election if he loses the high-stakes vote later tonight and has threatened to remove the whip from — essentially fire — any rebel Conservative MP who defies the government and votes with opposition lawmakers.

One of them, former treasury chief Philip Hammond, said he believed that between opposition lawmakers and rebel Conservatives, they have the numbers to vote for a delay to the Brexit deadline.

The Prime Minister suffered an embarrassing blow even before the vote when Conservative MP Phillip Lee defected to the Liberal Democrats right as Johnson was addressing the House on Tuesday afternoon. Lee's dramatic defection meant that Johnson's working majority in Parliament dropped from 1 MP to 0.

Johnson has been adamant he does not want to take the possibility of no-deal Brexit off the table. He believes the threat of no-deal makes his negotiation position stronger. Leaving without an agreement would be hugely disruptive for the British economy, but it would also hurt the rest of the EU.

The Prime Minister has been trying to renegotiate the withdrawal deal agreed with the EU by his predecessor Theresa May, demanding the removal of the so-called Irish backstop.

The clause is designed to prevent the reintroduction of border infrastructure between the Republic of Ireland, which will continue being an EU member, and Northern Ireland, which is part of the UK. The EU has so far refused, saying Johnson has not been able to offer a workable alternative.