Deal or no deal?
It would also raise the possibility of a delay to Brexit, with further votes on leaving without a deal and postponing Britain's departure date set for later in the week if May's deal falls.
Supporters say it allows Britain to control immigration and take advantage of global opportunities, striking new trade deals with the United States and others while still keeping close links to the European Union, which, even without Britain, would be a single market of 440 million people.
"Discussions are ongoing between ourselves and the EU", May's spokesman told reporters, insisting that Tuesday's vote would take place as planned.
Thursday's votes were a moment, it said, "for parliament to declare it wants an extension of article 50 so that, after two-and-a-half years of vexed negotiations, our political leaders can finally decide on what Brexit means".
"The government has been defeated again by an enormous majority", opposition leader Jeremy Corbyn said.
An picture taken in London on March 12, 2019 as an arranged illustration shows printed copies of the legal opinion on the joint instrument and unilateral declaration concerning the withdrawl agreement published by Britain's Attorney General Geoffrey Cox on Mach 12, 2019.
The document says that "persistent failure" to comply with a ruling "may result in temporary remedies".
"No deal must be taken off the table. The deal the UK Parliament will vote on tonight is the final one", he said.
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Up to 200 Labour MPs abstained from the referendum vote while 25 voted to support it.
The scale of the defeat means there is no way of knowing how, or even if, Britain will leave the world's largest trading bloc this month.
"I would hope it will be put to bed and we can all face up to the reality of the position and the opportunity that we have", Mr Johnson told MPs before the vote.
At a joint news conference with European Commission head Jean-Claude Juncker late on Monday, May announced three documents aimed at addressing the most contentious part of the exit deal she agreed in November - the Irish backstop.
While much of this will, inevitably, come down to a war of words and whether small gestures can change the perception of key voters in the Commons, Theresa May has already been dealt a blow by her own Attorney General, Geoffery Cox, this morning, who wrote in a letter to her that "The legal risk" to having the backstop "remains unchanged". The EU Commission said it would "expect a credible justification" for the postponement.
In essence, the assurances give the United Kingdom a possible path out of the backstop through arbitration and underscore the EU's repeated pledges that it does not want to trap the United Kingdom in the backstop.
But, if MPs once again reject the PM's deal, May has warned a longer extension will be necessary to work out what to do instead.
The pound fell sharply from recent highs plunging more than one per cent on the day against the euro. Afterward, hard-core Brexit supporters in May's Conservative Party and the prime minister's allies in Northern Ireland's Democratic Unionist Party both said they could not support the deal.
Charles Walker, a Conservative MP, said that a general election was now the way out of the impasse.
"We are in a very hard position", he said.