British Prime Minister Theresa May has three sitting days to return to parliament with a "Plan B".
Jeremy Corbyn is May's political opponent as leader of the Labour Party, and he has put up a vote of (no) confidence in May's government that is expected to take place in just one-day's time (Wednesday).
May, an initial opponent of Brexit who won the top job in the political turmoil that followed the referendum vote, will speak to the opposition Labour party, the Northern Irish DUP and her own lawmakers.
The British government and European Union leaders say their agreement is the best compromise available, and despite her historic defeat, May said it remained the only option. That would be the "disorderly Brexit" scenario.
Criticism of the deal is focused on an arrangement to keep open the border with Ireland by aligning Britain with some European Union trade rules, if and until London and Brussels sign a new economic partnership which could take several years. But extending by more than a few months would be a bigger ask.
May lost the vote on approving her Brexit deal by 432 to 202.
But finding a consensus on that direction is fraught, not least because public opinion is also split, says John Curtice, a politics professor at the University of Strathclyde.
Mike Cherry of the Federation of Small Businesses said many of its members would struggle to survive should deadlock in parliament lead to the country crashing out of the EU. Any alternatives, including calling a second referendum, would likely require the European Union to extend the March 29 departure deadline.
Her EPP boss, Manfred Weber, showed more exasperation than admiration.
While showing respect for the result of the vote, Dutch Prime Minister Mark Rutte said on Tuesday evening that the British parliament's rejection of the Brexit deal does not mean a no-deal situation and that "the next step is up to the United Kingdom".
Around 45 Labour MPs gathered outside Parliament this morning to launch their new campaign, holding a banner reading: 'Labour MPs, MEPs, conference and members agree - The people should have the final say'.
The last time an worldwide treaty was defeated by the British parliament was in 1864, when an extradition treaty with Prussia was voted down, according to the Hansard Society.
She is expected to do so - Conservative MPs and Tory allies the DUP are highly unlikely to join Labour in voting against her and the Government. "No such alternative deal exists".
This is a kind of safety net created to avoid physical border checks between Northern Ireland, which is part of the United Kingdom, and the Republic of Ireland, which is a member of the EU. It only comes into effect if the two sides are unable to negotiate a new trade agreement by the end of 2020. Nothing about Tuesday's vote has altered that trajectory.
However, Corbyn has been reluctant to push for a second referendum that could split his base.
Belgium's Prime Minister Charles Michel said on Twitter his country was also stepping up its planning for a Brexit no-deal. If the party doesn't shift its position, pro-EU lawmakers are planning to propose their own amendment to the next Brexit vote. "She needs to come up with something different than that", he said.
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While Parliament can pass laws and express its wishes, it can't replace the government at the negotiating table.
She promised to hold discussions with MPs from across parliament to identify ideas "that are genuinely negotiable and have sufficient support in this House". "Consequently, the government will continue to intensify preparations for such an outcome", Ireland government said in a statement.