Jeremy Corbyn has insisted that Labour will not stand in the way of Brexit amid fresh attacks on his “lukewarm” campaigning before the referendum.

The Labour leader ruled out support for a second referendum on the terms of Britain’s withdrawal, adding: “You have to respect the decision people made.”

The position stands in contrast to that of the challenger to his leadership, Owen Smith. The former shadow welfare secretary wants the British people to have another chance to back the case to either leave or remain in the European Union once the government’s proposals for the future terms of trade are laid out.

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Corbyn, who until recently supported leaving the EU, said: “They [the people] clearly have said no. Is there a way of having a European Economic Area agreement, possibly via Norway and other countries? Yeah, there probably is.”

The Labour leader’s comments came as he was being accused by leading lights in the remain campaign of sabotaging their hopes of persuading the UK to support EU membership.

The head of the official remain campaign, former Labour election candidate Will Straw, said in an interview with the BBC for a programme on the referendum to be aired on Monday, that he felt “let down” by Corbyn’s “lukewarm” support in the referendum. Straw complained that it had taken him six months to secure a meeting with one of the leader’s advisers.

Former European commissioner Lord Mandelson also told the BBC that remain campaigners were left puzzling over whether Corbyn, 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?

Facebook Twitter Pinterest Peter Mandelson giving a speech for the Britain Stronger In Europe campaign in March 2016. Photograph: Ben Pruchnie/Getty Images

“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?”

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.”

Straw, who was executive director of the Britain Stronger in Europe campaign, told the BBC 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.”

The 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.”

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Mandelson, however, was also critical of David 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.

He said: “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.

“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.”

Meanwhile the former Liberal Democrat leader, Nick Clegg, told the BBC that in discussions before the 2015 general election, 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?’” the former deputy prime minister said.

“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.’”

Clegg added that he had no doubt that former justice secretary Michael 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 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.”