The Luxembourg politician told a European Parliament plenary that if the UK government fails to ratify the withdrawal deal -- an almost 600-page document negotiated with the EU -- before October 31 then it would crash out in a so-called hard Brexit.
"If the UK is a member on May 23, it must organize European elections, if it does not do so and the withdrawal deal is not ratified by May 22, the extension ends on May 31," he said, adding that the stipulation was founded on a legal basis.
The European Parliament becomes illegal if a member state fails to run in the elections and therefore has representation in the chamber. The UK has 73 MEPs.
May's deal has been shot down in the House of Commons, the UK's lower chamber of Parliament, three times. The Conservative leader was engaged in talks with the opposition Labour Party leader Jeremy Corbyn in a bid to strike a consensus that would command a parliamentary majority.
European Council President Donald Tusk said the UK should not be treated as a "second-class" member state during the extension period. "I am strongly opposed to the idea that during this further extension the UK be treated as a second category member state."
"I also advised members of the chamber not to let Brexit-fatigue affect their judgment.
"I know that on both sides of the channel everyone, including myself, is exhausted with Brexit. However, this is not an excuse to says 'let's just get this over with because we are tired'.
"We must continue to deal with Brexit with an open mind and in a civilized manner. Because whatever happens we are bound by common fate and want to remain friends and close partners in the future," Tusk added.
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