The European Commission ruled out any renegotiation of the Brexit agreement, dealing a blow to Theresa May who has vowed to extract more concessions from Brussels.
The British Prime Minister cancelled the "meaningful vote" on her Brexit deal but insisted she would go to Brussels and demand "reassurances" over the Irish border backstop to get the agreement through Parliament.
An EC spokesman said: "We have an agreement on the table. This deal is the best and only deal possible. We will not renegotiate. Our position has therefore not changed and as far as we're concerned the UK is leaving the EU on March 29, 2019."
Leo Varadkar, the Irish Prime Minister, also said that it would not be possible to renegotiate the Irish border backstop without opening up the whole deal for further talks.
"The Withdrawal Agreement including the Irish backstop is the only agreement on the table. It took over a year and a half to negotiate and it's not possible to reopen any aspect of that agreement without opening all aspects of it."
He added: "We have already offered a lot of concessions along the way. We ended up with the backstop with this Withdrawal Agreement because of all the red lines the UK laid down along the way."
Brussels sources have suggested that some placatory language could be negotiated in a separate document to the Brexit agreement, in a similar way to how the EU mollified France over fishing in British waters and Spain over Gibraltar.
But Varadkar said: "I have no difficulty with statements that clarify what's in the Withdrawal Agreement but no statement of clarification can contradict what's in it."
Simon Coveney, the Irish Foreign Minister, said: "A lot of the commentary, which has been emotive and inaccurate in relation to the backstop coming out of Westminster, has created the backstop as something that it's not.
"This is simply an insurance mechanism that kicks in if all else fails to protect peace and stable relations on the island of Ireland."
May phoned Arlene Foster, the leader of the DUP, who tweeted: "My message was clear. The backstop must go. Too much time has been wasted. Need a better deal. Disappointed it has taken so long for Prime Minister to listen."
Guy Verhofstadt, the European Parliament's Brexit coordinator, raged on Twitter: "I can't follow anymore. After two years of negotiations, the Tory Government wants to delay the vote.
"Just keep in mind that we will never let the Irish down.
"This delay will further aggravate the uncertainty for people and businesses. It's time they make up their mind!" he added.
When asked what the next move from Brussels would be, one EU diplomat said: "The ball is not in our court".
Another EU diplomat said Olly Robbins, May's chief Brexit adviser, has been sent back to Brussels to seek a "legally binding" commitment that the backstop will never be used.
He said the only reason why May was taking this gambit was because the numbers voting down her deal would have been so bad as to make her leadership untenable.
Donald Tusk, the president of the European Council, said that the EU-27 leaders will discuss Brexit, without May, on Friday on the first day of a summit in Brussels, but warned the bloc would not renegotiate the deal.
Tusk did say that the EU would be willing to look at ways to help May get the agreement through Parliament but cautioned the bloc would step up its no-deal planning.
@GaryLineker: "Brexit is just a game to politicians. A game to see who gets to be Prime Minister. They don't give two hoots about what it does to our country. Shameless."
@MaryRiddell: "Brexit is dead. Theresa May is pursuing her own survival and a failed prospectus. Labour should now change this tragic game and demand a #PeoplesVote. The nation is crying out for sense and leadership."
@jessicaelgot: "Looks like we could be in this for the long haul. In Downing Street briefing just now there were no guarantees of a vote before xmas, indeed no guarantees of one in January. No expectation of securing change needed by end of the week, even after dash round EU capitals & summit."
Brexit: What now
What is happening?
The UK is due to leave the EU on March 29, 2019. Prime Minister Theresa May and EU leaders have negotiated a withdrawal agreement that spells out the terms of the departure. The House of Commons was scheduled to vote on the deal today, but May delayed the vote. It was clear that she didn't have the votes needed for passage. Much of the anger is focused on provisions designed to prevent the re-implementation of physical border controls between Northern Ireland, which is part of the UK, and Ireland, a member of the EU. Under the backstop, the UK would remain part of the EU customs union if the two sides couldn't agree on another way to avoid a "hard border". MPs criticised the backstop because the UK couldn't leave the arrangement without the EU's consent, giving European negotiators leverage to demand future concessions. The backstop would also treat Northern Ireland differently from the rest of the UK.
What does this mean?
A delay will force the Government to try to renegotiate its departure or face two equally unpalatable choices: a "no-deal" Brexit that could bring havoc to the economy or call a second referendum, which could undermine faith in the country's democratic institutions and lead to mass protests. Analysts say May's Government is weakened - perhaps fatally.
What are the options?
The Prime Minister may ask EU leaders to accept changes to the deal when they meet this weekend in hopes of winning support in Parliament. EU leaders insist the withdrawal agreement can't be renegotiated, but the declaration on future relations is shorter and subject to future negotiations so amendments may be possible. But there is no agreement about what a better deal would look like. Many Brexiteers seek a clean break with the bloc and want to change the Irish backstop. Pro-EU MPs want a softer divorce - the so-called Norway option - that would keep Britain inside the EU's single market for goods and services. The EU might be open to this idea, but it would mean accepting the continued free movement of EU citizens into the UK, a red line for many Brexit supporters.
Is May's job in jeopardy?
May's hold on power has been weakened by her inability to bridge differences within her Cabinet and finalise a deal. Pro-Brexit rebels in her Conservative Party can trigger a vote of no-confidence in May's leadership if they win the backing of 48 MPs. If May lost the vote, the party would hold an election to choose a new party leader - a process that would take several weeks..
Is there an election on the cards?
The Labour Party has threatened to call for a vote of no-confidence in the Government, which could trigger a general election. But winning such a vote would require the support of some Conservatives, who may be unwilling to trigger an election that could well see them ousted. If May's Government lost a confidence vote, it would have two weeks to overturn the result with a new vote by MPs. If that failed, there would be an election, a process that takes five to six weeks.
Could the UK change its mind?
Yes. The European Court of Justice ruled that the UK can decide to remain without the agreement of the other 27 members. Supporters of a so-called People's Vote want a second referendum now that the costs and benefits of Brexit are better understood.
- Addtional reporting: AP