British Prime Minister Theresa May has deferred a parliamentary vote on her Brexit deal due to concerns over the backstop.
The crucial vote was due to take place in the House of Commons tomorrow.
Addressing the Commons, Mrs May said that she had listened to the concerns of critics. She also said that the government was stepping up preparations for a possible no-deal Brexit.
"The deal would be rejected by a significant margin ... we will therefore defer the vote scheduled tomorrow," she said.
Mrs May, who will meet her EU counterparts in advance of the European Council this week, said that no Brexit deal is available without a backstop, which is designed to avoid a hard border on the island of Ireland post-Brexit.
She said that people living on the Irish border do not want their everyday lives to change as a result of Britain's decision to leave the EU.
Mrs May also said it is clear that the backstop can only ever be temporary, and it is clear that the EU will not want it to come into use.
The speaker of the House of Commons, John Bercow announced afterwards that parliament would hold a three-hour emergency debate tomorrow on the deferral.
This evening a spokesman for European Council President Donald Tusk has said he would call a special Article 50 summit on Thursday involving the leaders of the EU27 following the deferal of the House of Commons vote.
The spokesman added: "There will be no renegotiation of the 25 November agreement, but the EU27 stands ready to discuss how to facilitate UK ratification."
He added: "We are in contact with the UK on how it wants to proceed. As time is running out, EU27 leaders will also use the European Council to discuss how to prepare for a no-deal scenario.
"[President] Tusk is consulting EU27 leaders to prepare for the European Council on Thursday."
Mr Tusk said in a tweet that the EU will not renegotiate the Brexit deal, including the backstop.
"As time is running out, we will also discuss our preparedness for a no-deal scenario", Mr Tusk added.
I have decided to call #EUCO on #Brexit (Art. 50) on Thursday. We will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification. As time is running out, we will also discuss our preparedness for a no-deal scenario.— Donald Tusk (@eucopresident) December 10, 2018
Mrs May told MPs that her Brexit deal is the right one, and that she believes it is still possible to win enough support in the House of Commons to pass it.
She asked a "fundamental question - does this house want to deliver Brexit?"
A government spokesperson said a date for a new vote would depend on when Mrs May's government gets the assurances it needs to satisfy parliament.
This evening, MPs approved an application for an emergency three-hour debate tomorrow.
The application was successfully moved by Labour leader Jeremy Corbyn and was supported by a handful of Tory MPs.
Earlier, DUP party leader Arlene Foster said she told Mrs May to scrap the backstop.
Just finished a call with the Prime Minister. 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.— Arlene Foster (@DUPleader) December 10, 2018
The four pro-Remain parties in Northern Ireland have issued a statement saying it is vital that the backstop arrangement be banked in any negotiations to follow today's developments at Westminster.
The leaders of Sinn Féin, the SDLP, Alliance and the Green parties said "there is a pressing need for the backstop as set out in the Withdrawal Agreement to be banked".
They also stated a no-deal situation would be "catastrophic for our economic and society".
Following Mrs May's statement to the House of Commons, Mr Corbyn said the British government had "lost control of events and is in disarray".
He said the current deal is "a bad deal for Britain, a bad deal for our economy and a bad deal for our democracy," adding that "our country deserves better".
Mr Corbyn called for a House of Commons debate over the next two days to set out the negotiating mandate that Mrs May would take back to Brussels.
He said if she is going back to Brussels to renegotiate, she must build a consensus in the house.
He said Mrs May is trying to buy herself one last chance to save this deal, but must make way if she does not take on board the fundamental changes required to the deal.
European Court of Justice rules UK can unilaterally revoke Article 50
Reports that Mrs May could postpone the vote came just hours after the top EU court ruled that Britain could cancel its notice to leave the EU, which it is due to exit on 29 March 2019.
In an emergency judgment, the European Court of Justice said: "The United Kingdom is free to revoke unilaterally the notification of its intention to withdraw from the EU."
The ruling is in line with an opinion delivered last week by a Court legal adviser.
That had boosted the hopes of British Brexit opponents that a new referendum could be held that would prevent Britain's scheduled departure on 29 March 2019.
Scotland's First Minister Nicola Sturgeon accused the British government of "pathetic cowardice" by postponing the key vote on Brexit.
"This is a watershed moment and an act of pathetic cowardice by a Tory government which has run out of road and is now collapsing into utter chaos," she said in a statement.
Leading Conservative Remainer Anna Soubry said on Twitter: "PM said there'd be no General Election and there was. PM said there'd definitely be a vote tomorrow and there won't be.
"PM said there definitely won't be a People's Vote. Get ready People's Vote, taking this matter back to the people is the only way out of this Brexit chaos."
PM said there’d be no GE and there was. PM said there’d definitely be a vote tomorrow & there won’t be. PM said there definitely won’t be a People’s Vote....get ready @peoplesvote_uk taking this matter back to the people is the only way out of this #BrexitChaos— Anna Soubry MP (@Anna_Soubry) December 10, 2018
A spokeswoman for the ECJ said: "In today's judgment, the Full Court has ruled that, when a Member State has notified the European Council of its intention to withdraw from the European Union, as the UK has done, that Member State is free to revoke unilaterally that notification.
"That possibility exists for as long as a withdrawal agreement concluded between the EU and that Member State has not entered into force or, if no such agreement has been concluded, for as long as the two-year period from the date of the notification of the intention to withdraw from the EU, and any possible extension, has not expired."
Today's ruling upheld a finding by ECJ advocate general Manuel Campos Sanchez-Bordona, who said last week that Article 50 of the Lisbon Treaty allows the "unilateral revocation of the notification of the intention to withdraw from the EU, until such time as the Withdrawal Agreement is formally concluded".
He rejected the contention that the mechanism for a member state to quit the trade bloc could only be reversed following a unanimous decision of the European Council.
Additional Reporting PA