She won some last minute concessions from European Union negotiators on Monday to try and get a majority of the House of Commons on board with her plan, but her own attorney general made it clear the changes were effectively cosmetic, and would not give the United Kingdom the legal power to cut loose of the European Union unilaterally if negotiations drag on.
The defeat came after May and European Commission President Jean-Claude Juncker announced changes Monday created to overcome lawmakers' concerns about provisions created to ensure the border between EU member Ireland and Britain's Northern Ireland remains open after Brexit.
MPs will now vote at 1900 GMT on Wednesday on whether Britain should quit the world's biggest trading bloc without a deal, a scenario that business leaders warn would bring chaos to markets and supply chains, and other critics say could cause shortages of food and medicines.
This is a breaking news update.
British Prime Minister Theresa May's revised Brexit deal appears headed for certain defeat in the House of Commons Tuesday evening after key opponents to the agreement signalled they can't support a revised version of the pact.
If this is rejected, MPs will vote again on Thursday on extending Article 50. 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.
"I profoundly regret the decision that this House has taken tonight", May said.
This comes after earlier in the day, the Daily Mail reported that British ministers are set to expose Wednesday morning the secret tariffs and plans regarding the Irish border that would become effective in the event of a no-deal Brexit scenario.
But May's entreaty fell on deaf ears among lawmakers whose support she needs.
Lawmakers defeated May's deal by a whopping 230 votes in January, but May hoped the changes she secured from the bloc would be enough to persuade many to change their minds.
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Prior to the vote, UK Prime Minister Theresa May agreed with European Commission President Jean-Claude Juncker on "legally binding changes" to the text of the withdrawal agreement with the European Union including to the controversial Irish backstop provision, which would be complemented with "meaningful clarifications" and "legal guarantees".
"In the light of our own legal analysis and others we do not recommend accepting the government's motion today", William Cash, a senior pro-Brexit Conservative Party lawmaker said.
May said documents to be added to the deal provided "legally binding" assurances that the backstop would be temporary and that Britain would have a way to get out of it if the European Union failed to negotiate in good faith. We have the Withdrawal Agreement.
The vote was a second attempt for May to gather the approval of Parliament on the terms by which Brexit will occur.
"We are not preparing for, we do not want a general election: our position is unchanged", the spokesman said. "If there is a solution to the current impasse it has to be found in London", the spokeswoman said. So those MPs who favour a further referendum need to note that it can not be fitted into a three-month extension period. The main opposition Labour Party also said it would reject the deal.
"The EU has done everything it can to help get the Withdrawal Agreement over the line". "The government's strategy is now in tatters". "Our "no-deal" preparations are now more important than ever before".
"Now is the time to come together to back this improved Brexit deal and to deliver on the instructions of the British people".
Following the defeat, the prime minister confirmed that MPs will now be able to vote on whether to oppose a no-deal Brexit. "There is no alternative".
The stakes could hardly higher - and the future looks uncertain - after May failed for a second time to get Parliament to sign off on a divorce deal that was the culmination of two years of tortured negotiations. "We still see around a 30% chance of a second referendum, either called by.May or backed by a majority of MPs to break the deadlock, but believe the chances of another election remain slim", Tombs said.