Top European Union official Donald Tusk has warned the "toughest stress test" of Brexit negotiations is still to come and suggested Britain could yet stay in the bloc.
With Theresa May hoping exit talks will move on to discussions about trade by the end of the year, the European Council president insisted it is "up to London" how the negotiations will end.
"With a good deal, no deal or no Brexit," Mr Tusk added, in a report to the European Parliament on last week's gathering of EU leaders in Brussels.
At the summit, the other 27 EU member states refused to sanction the second phase of Brexit negotiations - on a possible post-Brexit trade deal and transitional arrangements - but agreed to begin their own internal preparatory talks on the subject.
In his statement to MEPs, Mr Tusk hailed the EU27's unity over Brexit and urged countries to remain together during the next segment of talks in order to "protect our common interest".
"We have managed to build and maintain unity among the 27," he said.
"But ahead of us is still the toughest stress test. If we fail it, the negotiations will end in our defeat.
"We must keep our unity regardless of the direction of the talks.
"The EU will be able to rise to every scenario as long as we are not divided."
Senior UK politicians such as Nick Clegg and Tony Blair have called for a second EU referendum once the detail of Britain's future relationship becomes clear.
It is legally unknown whether Article 50 of the Lisbon Treaty - the legal mechanism for EU departure triggered by the Prime Minister in April - is reversible, but it has been suggested political will in both the UK and Brussels could see the decision revoked.
The UK Government has consistently ruled out holding a fresh vote on EU membership and insisted Britain will leave the bloc in March 2019.
Liberal Democrat leader Sir Vince Cable welcomed Mr Tusk's comments.
"No matter what Theresa May says there are still three options on the table. No deal, a deal, or no Brexit," he said.
"The EU have confirmed what we have been saying all along, if Brexit looks like a disaster we can call the whole thing off."