Shadow health secretary Jonathan Ashworth says he was just "joking around" after a leaked recording emerged in which he criticised Jeremy Corbyn and said Labour wouldn't get a majority in the election.
Mr Ashworth commented the civil service machine would "pretty quickly move to safeguard security" if Mr Corbyn became prime minister in the audio obtained and released by the Guido Fawkes website.
He also said MPs "f***** it up" in 2016 when they moved to oust Mr Corbyn, saying they "went too early".
Mr Ashworth named the leaker as Tory friend Grieg Baker, who used to be the chairman of the Canterbury Conservative Association.
In the conversation, Mr Ashworth was also blunt about Labour's chances in this election, saying the electoral map was "topsy-turvy because of Brexit and because of Corbyn".
He said: "Question is to Labour, gets itself half decent leader, next time round reverse and regain its traditional heartland seats.
"I just can't see how you [the Conservatives] don't have a majority to be honest."
He said it was "dire" for Labour in traditional areas like Ashfield in Nottinghamshire.
He said: "Outside of the city seats, if you're in small-town Midlands and the North, it's abysmal out there. They don't like Johnson, but they can't stand Corbyn and they think Labour's blocked Brexit."
On the chances of a Labour majority, he said: "I just can't see it happening.
"Johnson and CCHQ would have to massively f*** it up."
As the Tory on the other end of the line laughed, Mr Ashworth said: "That might not reassure you too much."
Speaking to Sky News, he said the leak had made him look silly and said: "This is a Tory friend, clearly not one anymore.
"I was trying to wind him up a bit, pull his leg. I look clearly daft.
"I'm sorry to all the Labour members who are campaigning so hard in this election."
He said the Conservatives were trying to use the clip to "deflect from the crisis in the NHS".
He added: "I was winding him up, trying to make those I know on the Tory side complacent.
"I was trying to wind my mate up - a bit like when football managers wind up the manager of the other team.
"With your mate it seems fine but when it gets leaked it looks terrible and I'm sorry."
Asked about Boris Johnson's responses on Twitter to his comments, Mr Ashworth said: "Johnson can't even look at the photo of the four-year-old toddler on the floor in a hospital.
"He needs to be held to account for the crisis in the NHS he has made."
Mr Johnson had tweeted: "Jonathan Ashworth is saying what hundreds of Labour candidates and millions of voters are thinking. Jeremy Corbyn is unfit to be PM because he is blocking Brexit."
James Cleverly, the Conservative Party chairman said the call was an "honest and truly devastating assessment" of Mr Corbyn's leadership by "one of his most trusted election lieutenants".
The Brexit Election on Sky News - the fastest results and in-depth analysis on mobile, TV and radio.
• Watch John Bercow with Dermot Murnaghan live from 9pm on Thursday
• Follow the Election Social show hosted by Lewis Goodall and BuzzFeed UK's Emily Ashton on Sky News' website, app and social media channels also from 9pm
• See the exit poll live at 10pm
• Watch [email protected] election special on Friday morning