A leading conservative European parliamentarian said Thursday it would be "unthinkable" for Britons to take part in the upcoming EU elections, reiterating that there should be no postponement of Brexit.

"When a country is leaving the European Union, and that is the will of the Brits, then they cannot have a say in the future development of this continent. Sorry," said Manfred Weber, head of the conservative European People's Party which includes German Chancellor Angela Merkel's Christian Democrats.

"And that's why participation of Great Britain towards the European elections is for me unthinkable. Unthinkable," he told reporters at a EPP meeting in the Polish capital Warsaw.

Weber, who leads the biggest party in the European parliament, is bidding to take over from Jean-Claude Juncker as head of the European Commission, the bloc's executive arm.

He stressed that he was against giving Britain any more time regarding Brexit because it would disrupt elections for the European parliament which are scheduled for May 26.

"I have no appetite for discussing any kind of prolongation of the process of Brexit because our problem is not time," he said.

"Our problem is the real and clear understanding from our British friends of what they want to have as an alternative to the membership of the European Union."

"Time is running (out). They (Britain) have to clarify...."

With just three weeks to go until the scheduled departure date of March 29, concern is growing about the possibility of Britain crashing out of the bloc after 46 years of membership with no deal in place.

Britain's attorney general said Thursday that talks between British and EU officials to break the Brexit deadlock will "almost certainly" last into the weekend.

A crucial parliamentary vote in London is scheduled for Tuesday.

If British Prime Minister Theresa May loses Tuesday's vote, MPs will then vote on Wednesday on whether to proceed without a deal.

If MPs reject that outcome, they would then vote on Thursday on whether to ask the EU for a delay.

The request for a delay would have to be accepted unanimously by all member states and Britain would have to leave the EU on March 29 if it is rejected.