Guy Verhofstadt has said he does not “know” if Brexit will go ahead.
Questioned on this during a visit to London, the leader of the liberal group in the European Parliament told reporters: “Ask Theresa May.”
Meanwhile, European Council President Donald Tusk said there was a “20 to 30%” chance Brexit would not happen.
But Home Secretary Sajid Javid told the BBC it was “still possible” to get Theresa May’s withdrawal agreement with the EU through Parliament.
The House of Commons has rejected it three times, with the deadline for Brexit being delayed from 29 March to 31 October.
Mr Verhofstadt, who is the European Parliament’s Brexit co-ordinator as well as leader of the Alliance for Liberals and Democrats in Europe, campaigned in London on behalf of the Liberal Democrats ahead of European elections on 23 May.
Asked whether the UK would leave the EU, he replied: “I don’t know. It’s a question to ask Mrs May at Westminster.”
He also said the Brexit process so far “had done more damage than has ever been predicted” and that “people can change their opinion”.
Meanwhile, Mr Tusk told the Polish newspaper Gazeta Wyborcza: “Today the chance that Brexit will not happen is, in my opinion, 20 to 30%. That’s a lot.”
But Mr Javid told the BBC’s Political Thinking with Nick Robinson podcast: “What would make me happy is that we get this deal through. What would make me happiest is that we do it with my colleagues.
“It is still possible to get this through with support from Conservative MPs and the DUP and support this deal as it is and without any further changes and any further compromises.”
If this did not happen, he warned, MPs could seek to revoke the Article 50 process by which the UK is leaving the EU, in order to stop an exit without a trade deal with the EU.
“I can absolutely see MPs trying to come together to pass legislation to force the government’s hand to try and revoke,” he added. “That would be an absolute disaster.”
Cross-party talks between the government and Labour have been taking place to try to solve the Brexit impasse.
However, Labour leader Jeremy Corbyn said on Thursday that discussions had been difficult and that “so far there have been no big offers”.
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