The European Court of Justice has said the UK can revoke withdrawal process
A vote in Britain's House of Commons on the country's Brexit deal, scheduled for Tuesday, is to be delayed.
British Prime Minister Theresa May has said the vote will not go ahead as planned.
Mrs May earlier held a conference call with her cabinet, fueling speculation as to the next move.
Mrs May had warned her MPs that failing to vote for the deal risked handing the keys to Downing Street over to British Labour leader Jeremy Corbyn and his party, who say they could be ready to form a minority government by Wednesday.
She will delay a vote on her agreement with Brussels, which was likely to bring a heavy defeat for her government.
She will now stage a last-ditch attempt to win concessions from EU leaders at a Brussels summit on Thursday and Friday.
The decision to delay the House of Commons vote came after days of denial from Downing Street and UK cabinet ministers that the parliamentary vote would be postponed.
Even as late as Monday afternoon, the British government insisted Tuesday's vote was going ahead as planned.
Mrs May spoke to Taoiseach Leo Varakar and European Council President Donald Tusk by phone on Sunday.
The Government has said Mr Varadkar and Mrs May discussed the current situation on Brexit, including the planned vote in Westminster on Tuesday.
They also discussed preparations for this week's European Council in Brussels on Thursday, where they will meet.
Mr Tusk tweeted that it was an "important" week for Brexit.
She also spoke with European Commission President Jean-Claude Juncker and German Chancellor Angela Merkel.
But speaking at the Royal College of Surgeons in Dublin on Monday, Mr Varadkar said this is the only deal the UK will get.
"What I can say is that the withdrawal agreement - including the Irish backstop - is the only agreement on the table.
"It took over a year and a half to negotiate, it has the support of 28 governments, and it's not possible to re-open any aspect of that agreement without re-opening all aspects of it".
But he said he is open to clarifications: "I've no difficulty with statements that clarify what's in the withdrawal agreement - but no statement of clarification can contradict what's in the withdrawal agreement".
It comes after the European Court of Justice ruled that the UK can unilaterally revoke Article 50, triggering the Brexit withdrawal process.
In its judgement, it said: "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 along 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".
"The revocation must be decided following a democratic process in accordance with national constitutional requirements".
It added that any such revocation would see the terms of a member state status unchanged and "brings the withdrawal procedure to an end".
Read the full judgment here
Meanwhile a group of British MPs delivered a sharply critical verdict on the Brexit deal over the weekend.
The cross-party Committee on Exiting the European Union claimed that the Withdrawal Agreement and Political Declaration do not offer enough clarity or certainty about the future.
The report raised concerns about the Irish border situation, including efforts to ensure there is no hard border.
The MPs stated: "In December 2017, we said that we did not see how it would be possible to reconcile maintaining an open border on the island of Ireland with leaving the Single Market and Customs Union, which would inevitably make the Northern Irish border the UK's customs and regulatory border with the European Union.
"Since then, we have seen no realistic, long-term proposals from the Government that would address this."
They also suggested that 'uncertainty remains' for EU citizens in the UK, and insist that it would be 'unacceptable' if a planned White Paper on future immigration policy is not published before Tuesday's 'meaningful vote' on the deal in the House of Commons.
There were two rallies in London on Sunday, with pro-Brexit supporters holding a "Brexit betrayal" march, while others turned up to hold a counter-demonstration.
Additional reporting: Sean Defoe and IRN