The EU’s chief Brexit negotiator Michel Barnier has warned the UK government that “time is very short” to conclude a deal with the bloc.
Mr Barnier, who was speaking at a summit of EU leaders in Brussels on Friday, said a "huge and serious" gap still remained between the UK and the other 27-member states, particularly on the issue of the Irish border.
Britain plans on agreeing a withdrawal deal on its future relationship with the EU by October, five months before the UK formally exits the bloc in March 2019.
At the second day of the summit, EU leaders asked UK prime minister Theresa May to give “further clarity” on what kind of deal her government wants.
A long-anticipated policy paper is expected from the British government in early July, which Mr Barnier said he hoped would “contain workable and realistic proposals”.
"But let me mention once again that time is very short. We want a deal and are working for a deal, but time is short," he added.
Speaking on Thursday, Mrs May said both sides wanted to work "at a faster pace", while adding that “very good progress” had been made.
"We are ready to intensify and accelerate the pace of negotiations, I want to see that from the European Commission and the European Union as well," she said.
But Jean-Claude Juncker, the European Commission’s president, said divisions within Mrs May’s government were making the prospect of reaching an agreement more difficult.
"I don't have to lecture Theresa May, but I would like our British friends to make clear their position,” Mr Juncker said.
"We cannot go on to live with a split cabinet. They have to say what they want and we will respond to that."
While Ireland’s prime minister Leo Varadkar said that the British side must be willing to give up some ground if a deal is to be reached.
"Any relationship that exists in the future between the EU and the UK isn't going to be one of absolute equals," he said.
Mrs May is set to meet her split cabinet ministers at Chequers, the country estate of the UK prime minister, next week in attempt to iron out their differences and produce a white paper shortly afterwards.
London is clear that it wants to maintain close security ties with Brussels after March 2019 with Mrs May warning that only the closest possible alliance would protect the safety of EU citizens at a working dinner on Thursday night.