Briton cannot "have the EU cake and eat it too" over Brexit, the European Council President Donald Tusk says.
Speaking in Brussels, Mr Tusk warned that the EU would not compromise on its insistence that freedom of movement will be a condition for Britain's access to the single market.
Prime Minister Theresa May said last week that the government would trigger Article 50 of the Lisbon Treaty - beginning formal negotiations between the UK and EU - by the end of March next year.
In a 52 percent to 48 percent vote in June's referendum Britain decided to leave the EU.
Mr Tusk will chair meetings of EU leaders negotiating Britain's exit from the 28-member bloc.
The process will take up to two years, involving complex debates about issues such as immigration and access to the European single market.
'Only salt and vinegar'
In his speech, Mr Tusk mocked a Brexit campaign promise that Britons could "have the EU cake and eat it too" - the idea that the UK might manage to keep trade benefits of EU membership while barring European immigrants and rejecting EU courts' authority.
"To all who believe in it, I propose a simple experiment. Buy a cake, eat it, and see if it is still there on the plate.
"The brutal truth is that Brexit will be a loss for all of us. There will be no cakes on the table. For anyone. There will be only salt and vinegar."
Britain's only real alternative to a "hard Brexit" is "no Brexit", Mr Tusk said.
Mr Tusk also suggested that Britain might ultimately decide not to leave the EU "even if today hardly anyone believes in such a possibility".
He said the choice would be the UK's alone to make whether Brexit "is really in their interest".
British officials have so far made no public comments on Mr Tusk's latest statements.