"What is worrying us in particular is the Northern Ireland question where we expect a substantial accommodation from the British side". "The clock is ticking", German EU Minister Michael Roth told his EU peers meeting in Brussels.
"We need now to be making substantial progress, but that is not happening".
London has committed to avoid a "hard border" with checkpoints between Northern Ireland and Ireland, which all sides agree is vital to maintaining the 1998 Good Friday peace accords.
But Britain has also said it will not enter into a customs union with the EU post-Brexit and has been urged to find a solution to reconcile the two positions.
Ms May admitted there were flaws in both the customs models being considered by the government, a view reinforced by Michel Barnier, the EU's chief negotiator, at a foreign affairs meeting in Brussels.
But the European Union has said London must come up with a solution for the Irish border conundrum and highlights that has not happened.
The European Union urged the United Kingdom to accelerate the Brexit - the exit from the EU if London is going to have time to complete the main part of this process this autumn.
With May's cabinet, her ruling Conservative party and the British split on the matter, that has proven hard to achieve and the prime minister has come under increasing pressure at home in recent weeks to make a decision on customs. Within the it trade with the EU after Brexit should be carried out under the same conditions, but London will be able to begin trade negotiations with countries outside the EU, for now, remaining a member of the EU, he has no right. Allow me to repeat myself: "we are not there yet", Barnier said on Monday, adding the outstanding issues, including the Irish border conundrum, were "very serious".
"I wish above anything that we get a concrete proposal".
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