Donald Tusk has told the UK: "I hope you decide to return to the EU one day."
European Council president Mr Tusk, European Commission president Jean-Claude Juncker, Irish premier Leo Varadkar and EU chief Brexit negotiator Michel Barnier were speaking at a Brussels press conference to mark the successful conclusion of Brexit talks with the UK government.
But Mr Tusk, an outspoken critic of Brexit, could not resist floating the idea of Britain returning if MPs decide to back Prime Minister Boris Johnson's agreement on Saturday.
At the end of his speech on Thursday, Mr Tusk said: “Now we are waiting for the votes in both parliaments.
“On a more personal note, what I feel today is, frankly speaking, sadness. Because in my heart, I will always be a Remainer.”
And he said: “I hope our British friends decide to return one day. Our door will always be open.”
Mr Juncker also had a message for the 48 per cent who voted Remain in the 2016 referendum.
To laughter, he said: “They were right.”
