Theresa May’s proposed new economic partnership with the EU “will not work”, the head of the European Council has said.
Donald Tusk said the plans risked undermining the EU’s single market.
He was speaking at the end of an EU summit in Salzburg where leaders of the 27 remaining member states discussed Brexit.
Mrs May said her proposals were the “only serious credible” way to avoid a hard border in Northern Ireland.
Responding to Mr Tusk’s remarks, she said: “Yes, concerns have been raised and I want to know what those concerns are.”
There was “a lot of hard work to be done”, she said, but added that the UK was also preparing for leaving without a deal.
Mrs May reiterated that she would not accept the EU’s “backstop” plan to avoid a hard border in Northern Ireland, and said the UK would shortly be bringing forward its own proposals.
The UK is due to leave the EU on 29 March 2019, and both sides are trying to reach a deal in time.
There is still no agreement on some issues, including how to avoid new checks on the Northern Ireland-Republic of Ireland border.
At a press conference, Mr Tusk said there were some “positive elements” in the UK’s blueprint for future relations with the EU, which was agreed by ministers at Chequers in July.
But, he added: “The suggested framework for economic cooperation will not work.”
Mr Tusk added that October would be the “moment of truth” for reaching a deal, and that “if the conditions are there” an additional summit would be held in November to “formalise” it.
German Chancellor Angela Merkel said “substantial progress” was needed on the UK’s withdrawal agreement by the next European Council meeting in October, with “still a large piece of work” on the separate issue of future trade relations with the UK.
The 27 remaining EU members were “united that, in the matter of the single market, there can be no compromises,” she said.
French President Emmanuel Macron said Brexit had been “pushed by certain people who predicted easy solutions”.
He added: “Brexit has shown us one thing – and I fully respect British sovereignty in saying this – it has demonstrated that those who said you can easily do without Europe, that it will all go very well, that it is easy and there will be lots of money, are liars.
“This is all the more true because they left the next day, so they didn’t have to manage it.”
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