The frontrunner to replace Jean-Claude Juncker as the next European Commission president has urged Labour to back Brexit and vote for Theresa May’s withdrawal agreement.

Manfred Weber, the lead candidate for EU’s dominant political group and an ally of Mr Juncker said it was “absolutely unacceptable” that Jeremy Corbyn’s party was opposing the government.

Labour says it cannot back the deal unless Theresa May agrees to soften Brexit and link the UK into the customs union and single market – as some other EU figures have suggested. The party says it will back a second referendum if it cannot secure these changes.

But Mr Weber, who leads the European People’s Party in the European Parliament, said on Tuesday: “This is the last chance to reach a reasonable agreement on Brexit. There departure date cannot be postponed – a two-month long technical transitional period is the maximum we can accept.

“Jeremy Corbyn and Labour should finally take responsibility for their country and support this compromise. Their purely tactical behaviour and electoral calculations are absolutely unacceptable.”

Barring a major upheaval at the coming European Parliament elections in May, Mr Weber is likely to become the next Commission president, which would see him deal directly with the UK.

The vast majority of Labour MPs are expected to vote down May’s Brexit deal at the behest of the leadership later on Tuesday, with just a handful defying Mr Corbyn and supporting the exit plan.

Without Labour votes it would be very difficult for the Government to pass its Brexit deal – with many Tory Brexiteers still opposed and the DUP yet to come onside.

Other EU figures have been more supportive of Labour’s position in recent weeks. Udo Bullmann, the leader of the socialist group in the European Parliament, praised Mr Corbyn in February after his party backed a second vote.

“That's the announcement we were waiting for: Jeremy Corbyn’s plan or second referendum. UK Labour on the offensive,” Mr Bullmann said.

Manfred Weber leads the European People's Party, the largest group in the European Parliament (EbS)

European Council president Donald Tusk and others have also called on the UK government to consider Labour’s proposals for a softer Brexit in order to compromise and win their support.

Theresa May has refused to adopt any of the Labour proposals, however – for fear they would turn yet more of her MPs against her.

If MPs reject the Brexit deal on Tuesday evening the will be asked to vote on ruling out a no-deal exit on Wednesday, and then whether to ask for an extension of Article 50 on Thursday.