Lord Mandelson has accused party leader Jeremy Corbyn of sabotaging the Remain campaign to keep Britain in the EU.
The head of the official Remain campaign, former Labour election candidate Will Straw, also said he felt “let down” by Corbyn’s “lukewarm” support in the referendum, complaining that it took six months for him even to secure a meeting with one of the leader’s advisers.
Their comments came in a documentary called Brexit: The Battle for Britain which airs today on BBC Two.
Following the Leave victory on June 23, Corbyn came under attack from within Labour ranks for what was widely seen as a half-hearted campaign, in which he refused to join politicians from other parties on Britain Stronger In Europe platforms, opting instead to work with the single-party Labour In group.
Former European commissioner Lord Mandelson said Remain campaigners were left puzzling over whether the Labour leader - who told a chat show during the campaign that he would only rate the EU seven out of 10 - really wanted Britain to stay in the 28-nation bloc.
The Labour peer told the programme, Brexit: The Battle for Britain, “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.” 
Lord Mandelson added: “We were greatly damaged by Jeremy Corbyn’s stance, no doubt at all about that.
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“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.”
Straw, who was executive director of the BSIE campaign, told the programme: “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 yes.”
Shadow foreign secretary Emily Thornberry defended Corbyn’s campaign, telling the 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.”
Mandelson was also critical of Cameron’s handling of the referendum, accusing him of “holding back” the Remain campaign because of his unwillingness to go on the attack against fellow Tories.
“All the time we were being held back because the prime minister just simply didn’t want - and I completely understand why - to deepen the chasm that had broken out in his own party,” said Lord Mandelson.
“He thought that at the end of the day after he’d won the referendum he would have to bring everyone together and he didn’t want to ... poison the atmosphere any more. I said to George Osborne, we feel like sometimes we were taking a spoon to a knife fight.”
The claims come after Corbyn said Britian must abide by the referendum result.
Labour must “abide by” the decision of voters who backed Brexit in the European Union referendum, Jeremy Corbyn has said.
In one of the clearest indications yet that he will not seek to overturn the result of the June 23 poll, the Labour leader said voters had “clearly said no” and that Leave supporters would have to “work our way round that”.
His comments come days after he was repeatedly challenged by leadership rival Owen Smith to join him in demanding a second referendum on whatever deal emerges from Brexit negotiations.
In a head-to-head debate in Cardiff on Thursday, the Labour leader would say only that the issue of Brexit was likely to feature in the next general election.
Asked whether he thought that it would be undemocratic to seek to overturn the result, either through a second referendum or an election, Corbyn told the Huffington Post: 
“I think we’ve had a referendum, a decision has been made, you have to respect the decision people made. We were given the choice, we after all supported holding a referendum so we must abide by the decision.
“Does that mean that we don’t have a future relationship with the European Union? No, it means the opposite. There has to be a very strong relationship so I think there has to be a question of access to the single European market.”
He added: “It has happened: the Remain campaign didn’t get the majority, the Leave campaign did so we’ve got to work our way round that.”
In another interview, Corbyn denounced David Cameron’s controversial resignation honours list as “another nail in the coffin” of the House of Lords.
Cameron nominated 13 new life peers, including Conservative donors and Downing Street staff, while Corbyn also attracted criticism by nominating civil liberties campaigner Shami Chakrabarti shortly after she completed a report on allegations of anti-Semitism in the Labour Party.
The Labour leader told The Sunday Mirror: “I want the House of Lords abolished and replaced by an elected upper chamber.”