Brexit: Jeremy Corbyn undermined and sabotaged Remain campaign, claims Peter Mandelson

The former European Commissioner was being interviewed for a documentary examining why the British public voted to leave the EU

Lord Mandelson was interviewed for a BBC documentary on Brexit, to be broadcast on Monday night (PA)

Jeremy Corbyn has been accused by Tony Blair’s former right-hand man of “undermining” and “sabotaging” the campaign to keep Britain in the European Union.

The accusation by Peter Mandelson will infuriate Mr Corbyn’s supporters, who have repeatedly pointing out that a higher number of Labour voters than Tory voters, including a majority in Mr Corbyn’s own Islington North constituency, backed the unsuccessful Remain campaign – evidence, they say, that Mr Corbyn delivered.

However, the Labour leader is a long-standing opponent of EU membership, who is thought to have changed his mind on the issue relatively recently, and who refused to share a platform with David Cameron or other pro-Remain Tories during the referendum.

Lord Mandelson told the BBC: “It was very difficult to know what Jeremy Corbyn's motives were. Did he just sort of get out of bed the wrong side every day and not feel [in a] very sort of friendly, happy mood and want to help us?

“Or was there something deeper – did he simply not want to find himself on the same side as the Prime Minister and the Government? Or perhaps he just deep down actually doesn't think we should remain in the European Union? Who knows?”

He added: “We were greatly damaged by Jeremy Corbyn's stance, no doubt at all about that. Not only was he most of the time absent from the battle, but he was holding back the efforts of Alan Johnson and the Labour In campaign. I mean they felt undermined, at times they felt actually their efforts were being sabotaged by Jeremy Corbyn and the people around him.”

Lord Mandelson, who was one of the architects of Tony Blair's New Labour project, and is a former European Commissioner, was interviewed for the BBC2 documentary Brexit: the Battle for Britain, to be broadcast on Monday night, examining why the British public voted on 23 June to leave the EU.

Will Straw, a former Labour parliamentary candidate who headed the official Remain campaign, told the same programme that he felt “let down” by the “lukewarm” support the campaign received from the Labour leader. He said: “With just a couple of weeks to go, there were far too many people who didn't know Labour's position on the referendum. And I think that was because of a lack of concerted campaigning by the leadership over many months leading up to that point... I felt let down.”

After the referendum result, the presumption that Mr Corbyn had failed to give the Remain campaign adequate backing was the final trigger for the revolt by Labour MPs, during which more than 60 resigned from their frontbench positions.

But Emily Thornberry, who was promoted to the position of shadow Foreign Secretary after the mass resignations, defended Mr Corbyn. She told the same programme: “I think that all leading members of the Labour Party were out actively campaigning... and Jeremy played his part in that collective effort by doing a lot of media appearances, by doing a lot of, a lot of meetings up and down the country.

“He played his part and we all played our part in campaigning for that. I think that we are now going through a factious time in the Labour Party, clearly, but I don't think that it's appropriate for people to try to blame one individual.”

Lord Mandelson also accused David Cameron of “holding back” during the campaign for fear of widening the “chasm” within a divided Conservative Party.

The veteran Conservative and former Chancellor Kenneth Clarke had rather harsher words for the ex-Prime Minister, whom he accused of being “reckless and irresponsible” by calling the referendum in the first place.

One of the major setbacks for Mr Cameron was the unforeseen decision by his old rival Boris Johnson to throw in his lot with the Brexit camp. A former adviser of Mr Cameron has told the programme that the then-Prime Minister was given less than 15 minutes’ warning of Mr Johnson’s decision.