Donald Tusk, the European council president, has turned his fire on senior MEPs after he was accused of dividing the EU and prolonging the Brexit crisis for Europe by delaying it to 31 October.

The European parliament’s Brexit coordinator, Guy Verhofstadt, had warned during a Brexit debate that the EU27’s decision last week would “poison” the European elections, in which Britain would need to take part.

“Instead of sending May back to London with no extension or ultimately a very short one – a few days, a week – you gave her six months,” Verhofstadt said, after warning of a potential breakdown in EU unity over the issue. “In six months everyone knows, on 31 October, it is too near for a substantial rethink of Brexit and too far away to prompt any action … I fear it will prolong uncertainty and it will import the Brexit mess into the EU and poison the upcoming European election.”

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Verhoftstadt said the decision of the House of Commons to take an Easter break showed the talks between Labour and the Conservatives would merely “run down the clock”.

But, following a speech in which he had defended his right to dream of the UK changing its mind, Tusk gave a withering put-down of those who hoped to benefit from extra MEPs resulting from the UK’s withdrawal, including the liberal group led by Verhofstadt.

“I want to remind everybody that the UK has the right and obligation to take part in this election as long as it remains in the EU,” Tusk said. “This is not subject to negotiation. I also cannot agreed to accept a second category of membership. I understand party interests but they cannot overshadow the legal reality.

“Mr Verhofstadt was heartily and energetically applauded by Mr Farage. This is a good enough reason for you, Mr Verhofstadt, to deeply rethink and reformulate your argumentation.”

Earlier, Nigel Farage, who launched his new Brexit party on Monday, had told MEPs the delay was a “betrayal”, adding: “I’m coming back, lots and lots of us are coming back.”

Tusk defended the six-month extension – half the length of the one he had recommended to the EU27 at a summit last Wednesday – on the grounds that it would give Brussels time to focus on its priorities while allowing Westminster to find a way to break the impasse or allow the British people to “rethink Brexit”.

Q&A What does the European Council president do? Show Hide The European Council president is appointed by the elected national leaders of the EU member states. Their job is to, in cooperation with the European Commission, lead the European Council's work in setting the EU's general political direction and priorities. They also represent the EU on foreign and security matters. The role has been held by Donald Tusk since June 2017. Photograph: Mateusz Słodkowski/Rex Features

“I know that, on both sides of the Channel, everyone, including myself, is exhausted with Brexit, which is completely understandable,” Tusk said. “However, this is not an excuse to say ‘let’s get it over with,’ just because we’re tired. We must continue to deal with Brexit with an open mind, and in a civilised manner”.

Tusk added: “During the European council one of the leaders warned us not to be dreamers, and that we shouldn’t think that Brexit could be reversed. I didn’t respond at the time, but today, in front of you, I would like to say: at this rather difficult moment in our history, we need dreamers and dreams.”

The European commission president, Jean-Claude Juncker, called on Britain to remain a cooperative and loyal member state during its extra period in the bloc. He also urged Westminster to come to a decision on the future urgently, warning that a further extension this autumn could not be guaranteed.

“31 October marks the new deadline for the orderly withdrawal of the UK”, Juncker said. “If the withdrawal agreement hasn’t been ratified by the British parliament by then, then there will be a hard Brexit that we’d like to avoid. Of course the UK can request to revoke article 50, that’s something that’s been made very clear, but that’s not my working hypothesis. And it’s not my working hypothesis either that beyond 31 October we’ll seen an extension again.”