Theresa May’s future as UK Prime Minister is in doubt after her deal to leave the European Union was crushingly defeated in Parliament on Wednesday.

Ms May’s controversial Brexit deal was defeated by 432 votes to just 202, leaving Britain in limbo over the biggest political and economic change for the country in decades.

The result means around a third of Ms May’s own Conservative MPs voted against her proposal, making it the worst parliamentary defeat for a government in recent British history.

The humiliating loss, the first British parliamentary defeat of a treaty since 1864, marks the collapse of her two-year strategy of forging an amicable divorce with close ties to the EU.

Labour leader Jeremy Corbyn promptly tabled a motion of no-confidence in the government after the vote, with the future of the Conservative government to be debated on Thursday (Australian time).

If a majority of MPs support the motion, it will trigger a 14-day countdown for Ms May’s government, or any other alternative government, to win a vote of confidence and assume power.

If no potential government is able to garner sufficient support, then an early general election would be called for no earlier than a further 25 days.

“It is clear that the House does not support this deal, but tonight’s vote tells us nothing about what it does support,” Ms May told Parliament, moments after the result was announced.

“Nothing about how – or even if – it intends to honour the decision the British people took in a referendum Parliament decided to hold,” she said.

Ms May was earlier heckled with calls from Opposition MPs for her to resign as she made her last bid to sell the unpopular deal.

The scale of the defeat effectively scuppers the Brexit withdrawal Ms May negotiated with the EU. She now has three business days to come up with an alternative plan.

After the vote, Ms May indicated there would be no deferral of the March 29 Brexit deadline.

Ms May’s spokesman told reporters that the deal could still form the basis of an agreement with the EU, but opponents disagreed.

“The EU will see that it must now offer better terms to the UK. If it does not, we must leave to trade on WTO terms,” David Jones, a Conservative pro-Brexit former minister, said.

The small Northern Irish DUP party, which props up Ms May’s minority government and refused to back the deal, said it would still stand behind the Prime Minister in the no-confidence vote.

The pro-Brexit Conservatives who were the most vehement opponents of her deal also said they would support her.

The EU said the Brexit deal remained the best and only way to ensure an orderly withdrawal.

Austrian Chancellor Sebastian Kurz said there would be no further renegotiation of the agreement and EU Commission President Jean-Claude Juncker said it would intensify preparations for a no-deal Brexit.

Donald Tusk, chairman of EU leaders, suggested Britain should now consider reversing Brexit.

“If a deal is impossible, and no one wants no deal, then who will finally have the courage to say what the only positive solution is?” he tweeted.

The British pound rallied more than a cent against the US dollar, on some expectations that the scale of the defeat might force lawmakers to pursue other options.

Ms May said she would reach out to opposition parties to forge a way ahead. But across the British political spectrum, opponents of her deal said it was dead.

Ever since Britain voted by 52-48 per cent to leave the EU in a referendum in June 2016, the political class has been debating how to leave the European project forged by France and Germany after the devastation of World War Two.

While the country is divided over EU membership, most agree that the world’s fifth largest economy is at a crossroads and that its choices over Brexit will shape the prosperity of future generations.

Barely two-and-a-half months before Britain is due to leave the EU, the defeat leaves open a range of outcomes, from resurrecting her deal, leaving the EU with no deal at all, or holding another referendum that could halt Brexit altogether.

-with AAP