The UK's approach to the next stage of Brexit negotiations seems to be based on "pure illusion", Donald Tusk says.
The European Council president told a news conference in Brussels that the UK was still trying to "cherry pick" its future relationship with the EU.
Mr Tusk said he could only go on media reports of Brexit talks at the PM's country retreat Chequers on Thursday.
Theresa May is set to deliver a key speech setting out British ambitions on Friday.
Mr Tusk, who is due to meet the PM the day before, said media reports suggested the "cake philosophy is still alive" in the UK.
He added: "If the media reports are correct I am afraid that the UK position today is based on pure illusion."
He went on to reject - as he has done before - any notion of the UK "cherry picking" aspects of its future relationship with the EU or being able to join a "single market a la carte".
The BBC understands that the 11 senior ministers at Chequers made a breakthrough on so-called "managed divergence", where the UK could select EU rules to stick to post-Brexit.
Mr Tusk said the EU would continue to be "extremely realistic" during the forthcoming negotiations.
The second phase of Brexit negotiations will cover transitional arrangements after the UK leaves and economic and security co-operation in the future.
Mr Tusk, who spoke at an informal meeting of 27 European heads of states and governments, said he would present draft guidelines on the future EU-UK relationship at a summit in March.
"Our intention is to adopt these guidelines, whether the UK is ready with its vision of our future relations, or not," he said.
"Naturally it would be much better if it were. But we cannot stand by and wait."
He said he hoped to have more clarity when he meets the PM in London next week.
The leaders also spoke about the EU's post-2020 budget, the composition of the European Parliament, Turkey and Syria.
The government is set to pursue a policy which puts the UK outside a customs union with the EU - but matching EU rules in some industries in an attempt to achieve "frictionless trade".
Health Secretary Jeremy Hunt, who did not attend the meeting of senior ministers, said despite "divergent views" there was a "central common understanding".
He said some sectors could align regulations with European regulations, adding: "But it will be on a voluntary basis, we will as a sovereign power have the right to choose to diverge."