EU President Donald Tusk warned on Tuesday that some of the bloc's leaders were not yet ready to back a Brexit transition deal at a summit in Brussels later this week.
After intensive talks at the weekend, British and European Union negotiators reached an agreement Monday on a 21-month transition phase after Britain leaves the bloc in March 2019.
"Yesterday our negotiators reached a solution on parts of the withdrawal agreement. Whether all 27 member states can welcome this at the European Council remains open," former Polish prime minister Tusk said in his invitation letter to the leaders.
"I still need a couple more hours to consult with some of the most concerned member states."
He did not name the countries that remained to be convinced.
Britain and the EU also agreed on a "backstop" solution for the thorny issue of the future of the Irish border, just weeks after Prime Minister Theresa May said no British premier could ever accept such an idea.
"As regards the most contentious issue, namely Ireland, Prime Minister May has reassured me that she accepts all options agreed in December to be on the negotiating table," Tusk said in his letter.
EU negotiator Michel Barnier said on Monday that the two sides had made a "decisive step" on finalising Britain's withdrawal treaty, which should allow the unlocking of talks on a future trade deal in April.
Tusk will also discuss the "negotiating strategy" for the trade talks with the EU leaders, a senior EU official said.