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.
However Irish premier Leo Varadkar, in Berlin for talks with German Chancellor Angela Merkel, warned there were still gaps between the two sides.
"I think we are seeing steady progress in the Brexit negotiations, but more needs to be done in the coming weeks and months to close the remaining gaps between the EU and the UK," he said.
In his letter, Mr Tusk welcomed the agreement in the withdrawal deal on future citizens' rights, saying it would ensure EU nationals were "fully protected from the consequence of Brexit".
On the issue of the Irish border, he said Mrs May's acceptance that the "back stop" option of "full regulatory alignment" between the North and the Republic remained on the negotiating table "bodes well for the rest of the negotiations".
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.
Michael Gove said he was disappointed with the European Union over its stance on fishing as he faced Tory discontent about the Government's deal with Brussels.
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 rose to voice their frustration.
Douglas Ross, the MP for Moray, said: "There's no way I can sell this deal in the transitional period as anything like a success to fishing communities in Moray, Scotland or the UK."
Mr Gove, a leading Brexit campaigner in the referendum told MPs: "There is disappointment in fishing communities - I know, as someone whose father was a fish merchant, whose grandparents went to sea to fish, I completely understand how fishing communities feel about this situation at the moment.
"I share their disappointment."
In a sign of Tory unease about the reaction to the deal, Mrs May met MPs with fishing ports in their seats to explain the Government's approach.
Opponents of the agreement are planning to dump fish into the River Thames opposite the Houses of Parliament on Wednesday in protest.