Labour grandee Lord Mandelson has accused party leader Jeremy Corbyn of sabotaging the campaign to keep Britain in the EU.
And the head of the official Remain campaign, former Labour election candidate Will Straw, said he felt "let down" by Mr 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 BBC2 programme on the EU referendum, which also features:
:: Nick Clegg claiming that Michael Gove was the source for The Sun's controversial front cover story suggesting the Queen backed Brexit - something the former justice secretary has denied;
:: Europhile former Cabinet minister Kenneth Clarke condemning David Cameron's decision to call a referendum as "reckless and irresponsible";
:: A senior Cameron adviser confirming the then PM had less than 15 minutes' notice of Boris Johnson declaring for Leave;
:: The same adviser saying Mr Gove had given Mr Cameron the impression he would play a low-key role in the Brexit campaign, leading to "a moment of surprise" in 10 Downing Street when he was named chairman of Vote Leave.
Following the Leave victory on June 23, Mr 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 (BSIE) 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.
"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."
Mr 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 Mr 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."
Lord Mandelson was also critical of Mr 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."
Mr Clegg said that in discussions before the 2015 general election, Mr Cameron had brushed off his warnings that holding a referendum would be a risk.
"I said to him 'I can't get my head round this European gamble you've taken, are you sure you know what you wish for?'" said the former deputy prime minister.
"And I remember at the time David Cameron sort of very breezily saying: 'Oh of course it'll be won, of course it'll be won.' I said: 'Well, I'm really not so sure.'"
The former Liberal Democrat leader said he had no doubt that Mr Gove was to blame for one of the most controversial moments of the campaign, when a discussion he had supposedly had with the Queen was anonymously leaked to The Sun.
"Michael Gove obviously communicated it, well I know he did, he gave this to The Sun," said Mr Clegg.
"I know that. He leaked that and I can see why he might think that's an interesting thing to do to try and drag the Queen into it, but it didn't happen."
:: Brexit - The Battle for Britain can be seen on BBC2 at 9pm on Monday August 8.