LONDON - Britain's already disorderly departure from the European Union turned even more chaotic Monday when Prime Minister Theresa May postponed a House of Commons vote on her Brexit withdrawal deal, an agreement that took months of tortuous negotiations with Brussels to conclude.

After four days of debate in the House of Commons and a panicky effort by the prime minister to sell the deal to an increasingly disapproving British public, lawmakers were set to rebuff May's withdrawal agreement. Defeat would force May out of Downing Street and possibly trigger the fall of the Conservative government.

Britain's May Postpones Crucial Brexit Vote

While May Monday insisted publicly the vote on the withdrawal agreement, which she has staked her credibility on, would go ahead, aides said that behind-the-scenes, Cabinet ministers implored her not to move ahead. They urged her to return to Brussels instead to try to secure more concessions before the House of Commons has the final say.

Britain's May Postpones Crucial Brexit Vote

They argued May was facing a parliamentary defeat of historic proportions and needed to roll the dice. But Plan B — returning to Brussels to reopen negotiations on the 585-page deal — looks doomed.

Britain's May Postpones Crucial Brexit Vote

On news of the postponement, the already anemic pound crashed to its lowest level against the dollar in 18 months; it also fell against the euro.

Speaking to a packed and rowdy House of Commons, May said she had "listened to what had been said" during three days of debate. The comments provoked laughter and jeering from lawmakers.

"If the vote went ahead, it would be lost by a large margin. So the vote will be deferred," she said. May said the deal in its broad outlines was the best one for Britain. She said she would meet EU leaders "in the days ahead."

Labour leader Jeremy Corbyn dubbed the postponement a humiliation. He said the government had "lost control of events." He told the House of Commons she should make way for a new government.

"The government is in disarray … and people are in a state of despair." Opposition leaders were reportedly discussing tabling either a motion of no confidence in the government or one censuring the prime minister.

A clearly irritated Speaker of the House of Commons, John Bercow, criticized the delay and manner of it being announced, calling the move after three days of debate "deeply discourteous." He raised some potentially difficult procedural hurdles the government will need to navigate before postponing the vote.

Immediately on learning of the postponement, the European Commission ruled out re-negotiations as did some exasperated European national leaders." This deal is the best and only deal possible. We will not renegotiate the deal that is on the table right now. That is very clear," said an EU spokesman.

"Our position has therefore not changed and as far as we're concerned, the UK is leaving the EU on the 29 March 2019. We are prepared for all scenarios." The European Parliament's coordinator with the EU's negotiators, Guy Verhofstadt, voiced frustration, saying "I can't follow this anymore." He added, "It is time they made up their minds."

May's contentious deal, which tries to square the circle between Britons who want to remain in the European Union and Brexiters who want a clean, sharp break, would see Britain locked in a customs union with the European Union for several years while it negotiates a more permanent, but vaguely defined, free trade settlement with its largest trading partner.

In the temporary customs union, Britain would be unable to influence EU laws, regulations, and product standards it would have to observe. It would be not be able to implement free trade deals with non-EU countries, and Northern Ireland would be treated differently than other parts of Britain in order to avoid customs and immigration checks on the border with the Republic of Ireland.

That provision angers the province's pro-British Unionists, who fear Northern Ireland's ties with London will be diminished.May's minority government relies on the backing of 10 Unionist lawmakers to get legislation passed. The Unionists voiced concerns that May is trying to play for time. Their leader in the House of Commons, Nigel Dodds, cast doubt on her credibility and said the postponement was a "humiliation and a shambles."