EUROPEAN Council president Donald Tusk has warned there is no guarantee EU leaders will accept the agreement on the terms of Britain's withdrawal when they meet in Brussels this week.
Mr Tusk has said that he still needs more time to consult with "some of the most concerned member states" ahead of their two-day summit starting on Thursday.
The leaders of the remaining 27 had been expected to rubber stamp the agreement – including the transitional arrangements after the UK leaves in March 2019 – finalised by the EU's chief negotiator Michel Barnier and Brexit secretary David Davis on Monday.
But in his letter formally inviting them to the meeting, Mr Tusk said: "Whether all 27 member states can welcome this at the European Council remains open. I still need a couple more hours to consult with some of the most concerned member states."
Failure to to secure agreement on the terms of the UK's withdrawal would be a bitter setback for Theresa May, casting major doubt on her goal of securing broad agreement on Britain's future relationship with EU, including a free trade deal, by October.
Downing Street insisted they had made "very good progress" in reaching agreement with the European commission on the transitional arrangements.
"The European Commission has been clear, as has the European Council, that getting a deal is in the interests of not only the UK but businesses and people across the EU," the Prime Minister's official spokesman said.
Earlier environment secretary Michael Gove admitted that he was disappointed that under the terms of the agreement the UK would effectively remain subject to the EU Common Fisheries Policy during the transition period.
The government insisted it has agreed "specific safeguards" with Brussels over the annual fishing negotiations in 2019 but in the Commons a series of Tory MPs voiced their frustration.
Opponents of the agreement are planning to dump fish into the River Thames opposite the Houses of Parliament on Wednesday in protest.
Meanwhile, a Commons report has warned that Britain and the EU risk sleepwalking towards a crisis unless greater emphasis is placed on security and policing links.
MPs said more attention needs to be given to complex technical and legal obstacles to striking a deal that would maintain the existing level of cooperation on tackling serious crime and terrorism.
The Home Affairs Committee welcomed ministers' objectives for a security treaty to replicate current ties on Europol, the European Arrest Warrant and data sharing.
But it accused the British government of complacency over the timetable and urged the UK and EU to be flexible in coming to an arrangement.
