Jeremy Corbyn made the comments in an interview with the BBC's Andrew Neill

The Labour leader said Brexit throws up an “enormous complication” about how to deal with the border between Northern Ireland and the Republic.

In an interview with the BBC’s Andrew Neil, he said an agreement must be reached on movement of people and goods between the north and south.

Asked if voters should be allowed a referendum on uniting Northern Ireland and the Republic, he replied: “That’s up to the people of Northern Ireland to decide. If the Northern Ireland Assembly wants to have one then they should be allowed to. “

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Mr Corbyn insisted he was “not in favour” of a second Scottish referendum but Westminster should not block requests from the Scottish Parliament.

“I don’t think Westminster should block it but I think there should be a serious discussion about the timing of it because if they referendum occurs during the Brexit negotiations it becomes a bit complicated.

“I would say that it shouldn’t take place, if it takes place, until after the Brexit negotiations have been completed.”

Mr Corbyn backed the PM’s decision not to push for continued membership of the single market.

He said: “I don’t think we can be members of the single market if we are not members of the European Union. This issue is one of access.”

Mr Corbyn said Labour would vote against the final deal in Parliament if it fails to meet the six tests set out earlier this week by his party, which includes demands for the ‘’exact same benefits’’ as the UK currently has as a member of the single market and customs union?

“We would vote against it,” he said.

But he denied the move would lead to the UK crashing out of the EU, insisting an agreement could be reached with the other 27 members.