European Council president Donald Tusk has demanded more clarity from Prime Minister Theresa May over her plans for Brexit - and again held open the possibility of the UK changing its mind.
Addressing MEPs in Strasbourg, Mr Tusk 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'?"
Mr Tusk insisted that the EU had not had a "change of heart" over Brexit, telling the British: "Our hearts are still open for you."
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."
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."
And European Commission president Jean-Claude Juncker told the parliament in Strasbourg that he hoped Mr Tusk's message "will be heard clearly in London".
Their intervention came as Boris Johnson has said the Brexit campaign's highly controversial claim that leaving the European Union would mean Britain gets £350 million extra a week to spend on the NHS was an under-estimate.
The Foreign Secretary claimed the official Vote Leave campaign could have used a higher figure as the UK's gross contribution would rise to £438 million by 2021, the last year of an expected transition period.
According to the Guardian, he claimed Britain's contribution to the EU budget was already at £362 million a week.
Mr Johnson told the newspaper: "There was an error on the side of the bus. We grossly under-estimated the sum over which we would be able to take back control."
He added: "As and when the cash becomes available - and it won't until we leave - the NHS should be at the very top of the list,"
The claim first attracted criticism during the referendum campaign, when Mr Johnson was travelling around the country in a Vote Leave bus emblazoned with the slogan "We send the EU £350 million a week let's fund our NHS instead".