Alastair Campbell has said he would "have to really consider about whether I can stay in the Labour party" if Jeremy Corbyn stays on as party leader.
The lifelong Labour tribalist, who helped steer the party to three successive general election victories, added that he would have to "think long and hard" about even voting Labour if Corbyn is leader at the time of the next election, which could come within months.
Campbell, who said that Corbyn is staying on out of "vanity" despite it "blatantly not working", also accused the Labour leader of deliberately sabotaging the Remain campaign.
In a wide-ranging interview with Christian Today - in which he repeatedly swore about the behaviour of "unprincipled" Boris Johnson who he said had "brought down" David Cameron and "unleashed" racism on the streets - Campbell said Corbyn needs to look into himself and decide to resign.
"If he doesn't go he doesn't go," Campbell said of the embattled Labour leader. "And if he doesn't go or he has another contest and he wins it I think I'd have to really seriously consider about whether I can stay in the Labour party. I couldn't ever vote Tory...[But] I'd have to think long and hard about voting to make Jeremy Corbyn prime minister when he doesn't even want it....I can see little point being a member of a party which has become a sect and a vanity project. I would rejoin if and when we become a serious credible party of opposition again."
Campbell praised members of the shadow cabinet for breaking the Labour habit of keeping leaders in place. "I'm really glad that the shadow cabinet has been ruthless because we've gone on too long thinking we never get rid of leaders," he said. "When it's gone on so long blatantly not working you have to do something."
The Labour strategist added: "I said when he was going for it that I thought it would be a disaster for us. I then decided after he got it, keep quiet, I don't want to become a rent-a-quote, just say he's crap. But I think that now, where we're in such volatile times when the country's leadership, it's just so obvious he can't do the job. Now it's been obvious but in a way people felt it doesn't matter so much at the moment. But we could be heading for a general election."
On the Remain campaign, which Campbell helped to coordinate behind the scenes, he said of Corbyn: "I think what was clear during the campaign, the more I was talking to the people involved, they felt very, very strongly he was at best fighting it with his hands voluntarily tied behind his back, and at worst actually sabotaging it...Either he is incompetent or it was deliberate."
Addressing the issue of Corbyn's large mandate from new party members, Campbell said: "Now it's true that he inspired all these young people and others to join the Labour party but the idea that politics started then...I wasn't doing as much leafleting and door-knocking as [Campbell's partner] Fiona during the campaign but when I did do it the people out doing it were the ones who had been in the Labour party forever, they weren't these new people."
Campbell said that Corbyn needed to reflect in a way that Campbell himself did when he resigned as Tony Blair's director of communications. "I know it's not directly comparable but I do remember when I decided to leave in 2003, where there just came a point where you sort of looked into yourself and [thought] can you do this any more."
Campbell discussed his own evolving relationship with religion during the interview, which will be published soon on Christian Today.
