European Council President Donald Tusk has said there is a "special place in hell" for "those who promoted Brexit without even a sketch of a plan of how to carry it safely",
He was speaking after talks with Irish premier Leo Varadkar in Brussels.
He said the EU would "insist" on the Irish backstop in any UK withdrawal deal to preserve peace.
But he and Mr Varadkar were preparing for the "possible fiasco" of a no-deal Brexit.
Mr Tusk's Twitter account tweeted the same words immediately afterwards:
Theresa May - who supported the UK staying in the EU during the 2016 EU referendum - is due to arrive in Brussels on Thursday to seek legal changes to the withdrawal deal she signed with the EU. She hopes these changes will help her get it through the UK Parliament.
Mr Tusk said that the other 27 EU members had decided in December that the withdrawal agreement was "not open for renegotiation".
He said: "I hope that tomorrow we will hear from Prime Minister May a realistic suggestion on how to end the impasse in which the process of the orderly withdrawal of the UK from the EU.... following the latest votes in the House of Commons."
Mr Tusk said the Irish border issue and the need to preserve the peace process remained the EU's "top priority".
"The EU is first and foremost a peace project," he said.
"We will not gamble with peace or put a sell-by date on reconciliation. This is why we insist on the backstop."