"It is clear that the house does not support this deal, but tonight's vote tells us nothing about what it does support," May said in the Commons immediately following the vote. She further noted that a confidence motion would be voted on the following day.
Labour Party leader Jeremy Corbyn called the vote "a catastrophic defeat for this government."
"No deal must be taken off the table, a permanent customs union must be secured, and people's rights and protections must be guaranteed," Corbyn said. "Her governing principle of delay and denial has reached the end of the line. she cannot seriously believe that after two years of failure, she is capable of negotiating a good deal for this country."
Corbyn subsequently called May's government "incompetent" as he called for a vote of no confidence in that government.
Scottish National Party (SNP) leader in Parliament, Ian Blackford, called for the immediate suspension of Article 50, which sets the March 29 deadline.
Tuesday's vote was originally scheduled for early December 2018, but postponed by May once it became clear she stood no chance of getting it past the Commons. The deal, hammered out behind closed doors, without even her own Brexit Minister present, caused the resignation of several government ministers and widespread condemnation, including being found in contempt of Parliament, as it failed to settle several of the major items of contention she had promised would be settled. She ultimately faced down an internal revolt in the Conservative Party, surviving an attempted vote of no confidence on December 13.
The most sensitive is the so-called "Irish backstop," which avoided the question of a "hard" border across the island of Ireland between the Republic of Ireland in the south, and Northern Ireland in the north, by including the latter in the EU customs union. May's Democratic Unionist Party allies from Northern Ireland have dismissed the backstop as "toxic," as it separates the territory from the rest of the UK. That poses a danger to her government, which rules only with a supply and confidence agreement from the DUP. If the unionists pulled their support, May's government could crumble.
The agreement having failed to pass, May now has three days to submit an alternative, as per the closely-passed amendment passed last week. If May and Parliament can't agree on a deal with Brussels by March 29 and no other steps are taken, all ties between the UK and European Union will be cut overnight, meaning a no-deal exit, which May has warned will hurt the nation.
EU leaders have indicated Brussels intends to budge very little on the agreement hammered out between them and May behind closed doors last autumn. German Foreign Minister Heiko Maas told the Guardian Tuesday "if it goes wrong tonight, there could be further talks," but also dismissed the possibility that the deal could be fundamentally overhauled, noting that "if there were a better solution, it would already have been put forward."
There are a couple of options about what might happen next. May could resign, as did her predecessor, David Cameron, after he lost the initial Brexit vote on June 23, 2016; the DUP could pull its support from the government, forcing a general election; or Labour could call a vote of no confidence, which would give it the chance to form a government if successful. Another possibility is a second Brexit referendum — an option favored by many Labour MPs, but which has been dismissed as infeasible before the March 29 departure date by many observers.
Corbyn has previously opposed a renegotiation of the deal and favored a total departure from the EU, but the Labour MP said in Parliament Tuesday, "If Parliament votes down this deal, then reopening negotiations should not, and can not, be ruled out."