The President of the European Union made the remarks ahead of the EU's quarterly summit in Brussels, becoming the latest in a series of leaders to say that Britain could still have a future in the bloc.
EU president Donald Tusk on Thursday said Brexit could be reversed as leaders gathered for a Brussels summit amid growing confidence in a future without Britain.
Embattled Prime Minister Theresa May will make EU leaders an offer on the rights of expats after Britain's withdrawal as she tries to convince them she still has a grip after her election meltdown.
"Some of my British friends have asked me whether Brexit could be reversed, and whether I could imagine an outcome where the UK stays part of the European Union," Tusk told reporters.
"I told them that in fact the European Union was built on dreams that seemed impossible to achieve, so who knows?" the former Polish premier said.
TRT World's Kim Vinnell has more on the summit from Brussels.
Tusk, who has repeatedly said Brexit benefits no one, especially not Britain, channelled former Beatle John Lennon as he became the latest in a series of EU leaders to suggest it was not too late to change tack.
"You may say I am a dreamer, but I am not the only one," he added with a broad smile, quoting Lennon's iconic song "Imagine."
Newly-elected French President Emmanuel Macron and German Finance Minister Wolfgang Schaeuble both said last week that the "door was open" for Britain to remain in the EU.
Tusk insisted the remaining 27 members had a renewed sense of optimism about the bloc's future after years of crisis and mounting anti-EU sentiment culminating in the Brexit vote.
Despite it being his 80th summit as premier or EU head, "never before have I had such a strong belief that things are going in a better direction," he said.
"Our optimism should still be extremely cautious but we have good reason to talk about it," Tusk said shortly before the summit opens at 1300 GMT.