Labour's "devastating" general election defeat could spell the end of the party, shadow health secretary Jonathan Ashworth has warned.
The Labour MP told a BBC Radio 4 documentary the party had to change or face up to 15 years out of power.
"Unless we do something quick this could be the end of the Labour Party in this country", he said.
Senior Labour and Tory figures give frank assessments of the 2019 campaign in the programme to be aired on Sunday.
Labour went down to its worst defeat, in terms of seats, since 1935, although its vote share was slightly higher than when it lost in 2010 and 2015.
Mr Ashworth had to apologise to his party two days before the election, after saying he did not believe it could win, blaming "the combination of Corbyn and Brexit".
'Voters didn't buy it'
He said he was "set up" by a Tory friend, who secretly recorded his comments and released them to the media.
"I obviously regret that and I still think about it all time," he told The Inside Story of Election 19 documentary.
But he added: "I was correct - that was the irony of the phone call."
The Leicester South MP is critical of Labour's decision to focus on claims the Conservatives were plotting to put the NHS on the table in trade talks with America.
"It was a legitimate point to make, but I think, politically, voters didn't buy it," he said, because it did not directly affect their lives.
He also claims there was a lack of communication and planning at the heart of Labour's campaign.
He said he was told about the party's free broadband pledge by a BBC Newsnight producer while he was waiting to go on air to talk about Labour's NHS policies.
"I knew nothing about it," he said, adding that it "was a week when we were trying to focus on the NHS and we were suddenly going off in another direction".
'A Brexit election'
He added that he did not think Labour should assume it would automatically return to power - and it needed to change.
"We could be out for another five to 10, 15 years," he said.
Jeremy Corbyn's former director of communications, James Schneider, also highlights a lack of trust between the key players in the campaign.
"John (McDonnell) and his shadow Treasury team did guard quite closely their big announcements," he told presenter Anne McElvoy.
"Over the course of 2019, and the disagreements on Brexit, the levels of trust within the operation had reduced very much and there was some operational scratchiness."
He said Labour's campaign slogan "it's time for real change" never "took off", adding "we never filled it in with content or with other meaning", giving the impression that the party was "throwing things at the wall".
The Conservatives honed their social media messages during the campaign before unleashing a barrage of "negative, nasty messages about Jeremy" in the final days, says Mr Schneider.
Isaac Levido, the Australian strategist who ran the Conservative campaign, said he had no regrets about any of the party's messages.
He describes a much-criticised social media video, edited to show Sir Keir Starmer apparently unable to answer a question on Labour's Brexit position, as "a little bit of fun".
"From time to time you need to do things that cut through," he added.
He also revealed that he had tried to maintain morale at Tory HQ by playing music, including One Day More, from the musical Les Miserables, on the eve of polling day.
"I think there was a bit of a sing-a-long. People were quite delirious by that point," he added.
Cabinet Minister Michael Gove is asked if it was a mistake for Boris Johnson not to be interviewed by the BBC's Andrew Neil.
"No, because we won," he replied.
"I'm sure Boris would have done brilliantly, but with the best will in the world and I am huge fan of Andrew Neil, the purpose of running an election campaign is to win so that you can govern the country well, not to agree to every broadcast bid."
Lib Dem leadership contender Layla Moran said her party failed to properly explain its policy of cancelling Brexit if it won the election.
"It made us look arrogant, and it made us look stupid," she says.
The Inside Story of Election 19 is on BBC Radio 4 at 1330 GMT, presented by Anne McElvoy and produced by Peter Snowdon.