The EU's chief Brexit negotiator has said Brussels "profoundly regrets" the Commons vote on Theresa May's Withdrawal Agreement after two years of negotiation "based on the red lines of the British government".
Michel Barnier said the risk of a 'no-deal' Brexit is higher than ever before.
He said the British government must say what it wants to do next, but it is too early to assess the consequences of last night's vote, which saw Mrs May's government suffer the largest defeat in modern British history.
He added that the backstop must remain and be credible in any Brexit deal, but the EU was ready to discuss a different deal if the UK changes its existing "red lines".
Mrs May will face a confidence vote today after MPs overwhelmingly rejected her deal to leave the European Union, leaving Britain with no plan as it hurtles towards Brexit on 29 March.
Speaking in the European Parliament in Strasbourg, Barnier said the vote showed the "political conditions" were not yet present in Westminster to ratify the agreement.
"It is up to the British authorities today or tomorrow to assess the outcome of this vote and up to the British government to indicate how we are going to take things forward on March 29 to an orderly withdrawal," he said.
The EU has warned that the vote raises the risk of a hugely disruptive "no deal" Brexit where Britain could sever ties with its biggest trading partner overnight.
European Commission President Jean-Claude Juncker urged London to "clarify its intentions as soon as possible", warning that "time is almost up."
Mr Juncker said the treaty was the only way to ensure an orderly withdrawal of the UK from the EU, while European Council President Donald Tusk said the EU would remain united, but that preparations for a 'no-deal' would continue.The EU's Chief Brexit negotiator, Michel Barnier,
The Irish Government said it would now intensify preparations for a "disorderly Brexit".
Mrs May struck a conciliatory tone after the vote, promising cross-party talks to try to salvage a workable Brexit deal before returning with a new plan next Monday.
She expects to win the confidence vote scheduled for around 7pm.
It was tabled by opposition Labour leader Jeremy Corbyn, who wants to force a general election.
While her own Conservative MPs and her allies in the DUP have led the charge against her Brexit deal, they do not want a Labour government.
Most MPs, including Mrs May, opposed Brexit ahead of the 2016 referendum, which has caused bitter divisions across the UK.
Nearly three years later, they still cannot decide what to do, with MPs criticising Mrs May's deal both for keeping Britain too closely tied to the EU, and for not keeping it closer.
Former foreign minister and leading Brexit supporter Boris Johnson said the vote gave Mrs May a "massive mandate" to renegotiate her deal with the EU.
DUP leader Arlene Foster, whose ten MPs prop up Mrs May's government in the Commons, also called for "fundamental change" to the Brexit deal.
"We will give the government the space to set out a plan to secure a better deal," she said.
But, speaking ahead of today's vote, German Foreign Minister Heiko Maas ruled out any substantial changes to the withdrawal agreement - a message reinforced by Mr Juncker.
"It is the only way to ensure an orderly withdrawal of the United Kingdom from the European Union," Mr Juncker said.
The EU was expecting the Withdrawal Agreement to be rejected by the House of Commons, but officials were still shocked at the scale of its defeat.
German finance minister and vice chancellor Olaf Sholz described the result as a bitter day for Europe, and said a no-deal Brexit would be the least attractive choice.
Austrian Chancellor Sebastian Kurz said there would be no renegotiation of the Withdrawal Agreement.
French President Emmanuel Macron is reported to have said that Britain would be the biggest loser in a no-deal Brexit.
The EU believes the House of Commons must now find a way through the tangle of options, but the scale of the defeat means that even minor adjustments to the treaty would not have overturned the result.
An extension to Article 50 now looks a more likely scenario, but the EU will want to know exactly what it is being extended for.
Increasing numbers of pro-European MPs are calling for a second referendum as the way out of the deadlock, with an option to cancel Brexit altogether.
Mr Tusk tweeted after the vote: "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?"
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?— Donald Tusk (@eucopresident) January 15, 2019
Speculation is also growing on both sides of the English Channel that Mrs May could ask to delay Brexit, although she denies this.
British businesses urged politicians to unite.
"Financial stability must not be jeopardised in a game of high-stakes political poker," warned Catherine McGuinness, policy chair at the City of London Corporation, the body governing the British capital's financial district.
Additional Reporting Tony Connelly