EU President Donald Tusk has said the only alternative to a so-called "hard Brexit" which would see Britain pull out of the bloc's single market and impose tough immigration controls, is "no Brexit".
"The brutal truth is that Brexit will be a loss for all of us," he said in a speech at Brussels think tank the European Policy Centre.
"I think it is useless to speculate about 'soft Brexit'," he added.
"In my opinion, the only real alternative to a 'hard Brexit' is 'no Brexit'. Even if today hardly anyone believes in such a possibility," said the head of the European Council, which groups the 28 EU leaders.
He added that other EU leaders would be sympathetic if the UK Government reversed the Brexit decision.
"If we have a chance to reverse this negative process, we will find allies," he said.
Mr Tusk said the Article 50 process could be halted by British Prime Minister Theresa May's government even once it had been triggered.
The sterling has fallen dramatically as British leaders bicker over how to leave the European Union, with many fearing a "hard Brexit" would mean Britain withdrawing entirely from Europe's single market, and having to negotiate new trade arrangements, in order to impose strict immigration controls.
EU leaders have said Britain must accept free movement of people if it wants continued access to the single market and have warned the Brexit negotiations will be tough.
Ms May announced earlier this month her government will trigger those negotiations by the end of March, putting the country on course to leave the EU by early 2019.
Mr Tusk said: "We will conduct the negotiations in good faith, defend the interests of the EU 27, minimise the costs and seek the best possible deal for all.
"But as I have said before, I am afraid that no such outcome exists that will benefit either side."
