Alastair Campbell says he no longer wants to be a Labour member, claiming the party is facing an “existential crisis” due to poor leadership.
Tony Blair’s ex-spin doctor had planned to challenge the decision to kick him out after he admitted voting Lib Dem.
But he said he did not want to return because Labour had been “taken over” and it was “time to stop pretending” it was the party it used to be.
Shadow climate minister Danielle Rowley said his decision was disappointing.
Mr Campbell told BBC Radio 4’s Today programme Mr Corbyn “has not led on Brexit” or “the anti-Semitism issue”, and the leadership “kid themselves that there’s a policy agenda that the country out there is even aware of”.
“What you have to do in opposition to win against a ruthless Tory Party, you have to do far more that is being done now. And we have to be honest about that.”
He said he feared Prime Minister Boris Johnson was trying to “clear the decks towards a general election” because he believed the Labour leader’s “weakness” made a Conservative victory more likely.
“He thinks, probably rightly, that the country has decided it will not put Jeremy Corbyn into office,” Mr Campbell continued.
“I think there is a danger that we’re going to be destroyed as a serious credible political force unless we face up to the reality of what’s going on.”
Mr Campbell said he voted Lib Dem in May’s European elections because he wanted to support an unequivocally pro-Remain party.
Labour said at the time his expulsion was “automatic” because he had admitted voting for another party.
Asked whether he would now join the Liberal Democrats, he said he was not “close to other parties” and had not yet decided who to vote for at the next election.
In an open letter to Mr Corbyn, published in The New European magazine, he said he hoped to one day rejoin a party “that genuinely appeals to the many not the few”.
Ms Rowley told Today Mr Campbell’s letter to the Labour leader was “disappointing”.
“The Labour Party has always been and remains a broad church – there’s lots of different viewpoints,” she said.
“I’d like it if he could get on board, but it seems he really doesn’t want to do that.
“If he is really interested in making sure the Labour Party is doing what he thinks it should be doing, why is he not having these conversations privately and trying to genuinely talk about his concerns, rather than going out and writing letters in the papers and doing it publicly?”
Who is Alastair Campbell?
Mr Campbell was a political journalist before coming to prominence in Whitehall as a key member of the Labour PM’s staff in 1994.
He served as Mr Blair’s chief press secretary until 2000 and was a controversial figure, heavily involved in policy, including over the Iraq War.
Since leaving government, he has opened up about his struggles with depression and alcoholism, and works with a number of charities.
He also campaigns for the People’s Vote and is editor-at-large of The New European magazine.