Britain will lose influence in international affairs and become a "second-rate player" after it leaves the bloc, EU Council President Donald Tusk said on Wednesday.
Tusk was giving a keynote speech at the College of Europe in the Belgian city of Bruges, reflecting on his two terms as President of the European Council.
Tusk said only a united Europe could confront an assertive China and play an effective global role.
"I have heard repeatedly from Brexiteers that they wanted to leave the European Union to make the United Kingdom global again, believing that only alone, it can truly be great...," Tusk said.
"But the reality is exactly the opposite. Only as part of a united Europe can the UK play a global role, only together can we confront, without any complexes, the greatest powers of this world. And the world knows it," said Tusk, who has chaired EU summits for the past five years and been an influential voice in European politics.
"I have heard the same in India, New Zealand, Australia, Canada and South Africa - that after its departure, the UK will become an outsider, a second-rate player."
Tusk has repeatedly voiced a wish for Britain to change its mind and stay in the EU.
"'Why are they doing this?' I was asked this regretful question everywhere I went", Tusk said. "One of my English friends is probably right when he says with melancholy that Brexit is the real end of the British Empire."
Tusk's successor, the Belgian Charles Michel, takes office as his replacement in 20 days, on Dec. 1.
Former Polish leader Donald Tusk has been serving as president of the European Council since 2014.