European Council President Donald Tusk raised the idea that Brexit might be called off in a tweet on Monday after Boris Johnson, a campaigner for Britain to leave the EU, resigned as foreign secretary.

Echoing a comment he made to reporters after the earlier resignation of Brexit Secretary David Davis, Tusk tweeted: "Politicians come and go but the problems they have created for people remain. I can only regret that the idea of Brexit has not left with Davis and Johnson. But...who knows?"

Tusk, a former Polish prime minister, is overseeing the negotiations with London on its departure and has been highly critical of Brexit. He has said in the past that Britons would be welcome to change their minds and stay in the European Union.