European Union President Donald Tusk said that the bloc's "hearts are still open" to Britain if it changes its mind about leaving, in a fresh suggestion that Brexit could be reversed.
Noting that Brexit was due to happen in March 2019 "unless there is a change of heart among our British friends," Mr Tusk told the European Parliament: "We on the continent haven't had a change of heart. Our hearts are still open to you."
Mr Tusk demanded more clarity from British Prime Minister Theresa May over her plans for Brexit .
He said: "If the UK government sticks to its decision to leave, Brexit will become a reality - with all its negative consequences - in March next year unless there is a change of heart among our British friends.
"Wasn't it David Davis himself who said 'if a democracy cannot change its mind, it ceases to be a democracy'?"
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The December European Council had agreed that "sufficient progress" had been made on the first phase of Brexit talks, allowing negotiations to move on to consider transition and a future deal.
Mr Tusk said: "As regards our future relations, what we need today is more clarity on the UK's vision.
"Once we have that, the leaders will meet and decide on the way the EU sees its future relationship with the UK as a third country."
Calling for continued unity among the remaining 27 members of the EU, he said: "The hardest work is still ahead of us and time is limited.
"We must maintain the unity of the EU27 in every scenario, and personally I have no doubt that we will."
European Commission president Jean-Claude Juncker told the parliament in Strasbourg that he hoped Mr Tusk's message "will be heard clearly in London".
