Alastair Campbell, the former No 10 director of communications under Tony Blair, has said he no longer wishes to be a Labour member, warning that Jeremy Corbyn is poised to lose the next election against Boris Johnson and destroy the party “as a political force capable of winning power”.

Campbell had planned to fight his expulsion from Labour for revealing that he voted Liberal Democrat in the European elections, but he has now written to Corbyn informing him: “With some sadness but absolute certainty, I have reached the conclusion that I no longer wish to stay in the party, even if I should be successful in my appeal or legal challenge.”

The former Labour adviser and campaigner for a “people’s vote” has written a lengthy missive to Corbyn, part-published in the Guardian and in full in the New European, setting out his view that the party will not win a majority against Johnson, who he says is “embarked on a crash and burn strategy” designed to win a mandate for hard Brexit at the ballot box.

“The culture you have helped to create has made the party one that I feel no longer truly represents my values, or the hopes I have for Britain,” Campbell wrote. “I see no strategy in place that remotely meets the electoral or policy challenges ahead. On the contrary, in so far as I ascertain a strategy at all, it is one that looks more designed to lose. I fear the country may already have decided that it does not intend to make you prime minister.”

Campbell has been appealing against his expulsion since he was ejected under an automatic process designed to deal with those expressing support for another party. It caused mixed reactions at the time, with Charles Clarke, a former home secretary, revealing that he too had opted for the Lib Dems in the European elections because of their pro-remain policy, and Tom Watson, the deputy Labour leader, saying the move to kick out Campbell was “spiteful”.

Others within Labour described the decision to expel Campbell as merely a matter of applying party rules consistently, while Momentum, the grassroots group of Corbyn supporters, celebrated his departure saying that “being kicked out of the party is the least he deserves” for his role in Blair’s government and the Iraq war.

In his letter, Campbell said a member of Corbyn’s team had more recently suggested there could be two possible routes to him being let back into the party, one of which was a suspension of his auto-exclusion during a review of the party’s whole disciplinary system, and the other some kind of public commitment to voting Labour at the next election.

However, he said he felt unable to give a commitment to voting Labour while his concern about Corbyn’s stance on Brexit was “still acute”.

“I do not know at this stage how I will vote at the next election and I have made this decision after discussing it with nobody apart from family and a small number of close personal friends,” he said, adding that this was not part of some bigger plan but a deeply personal decision.

“What I do know is that this is indeed a moment of real peril,” he wrote. “To have any chance of stopping Johnson and stopping a hard Brexit, you need to step up now, and signal leadership of the anti-Brexit, anti-populist cause, though it may be that loss of trust in your approach to Brexit means it is too late to win back many former supporters.”

Corbyn said at the weekend he was “raring to go” and ready for an election against Johnson, which could come as early as this autumn if the EU is not open to changing the withdrawal agreement and parliament blocks a no-deal Brexit.

Labour has changed its policy to call for a second referendum on any deal and to avoid no deal, but some Labour MPs are still pushing for a more unequivocally pro-remain position in all circumstances to head off the threat of the resurgent Lib Dems and win over anti-Brexit swing voters.

Since Johnson came to power, his government has enjoyed a modest boost in the polls, with some putting the Conservatives neck-and-neck with Labour and others putting the Tories ahead.

Campbell said he did not feel Corbyn’s shift towards supporting a second referendum went far enough, arguing: “Without real change, there will be nothing left to fight for, and that your place in history will be as the leader who destroyed Labour as a serious political force capable of winning power.

“With a government this bad, pursuing a ruinous form of Brexit that will so damage our economy, society and standing in the world, Labour should be poised to win an election,. If the public could see that clear, credible and coherent alternative across the despatch box, ably led, we would be. That the country does not see it is, I am afraid, very substantially down to you.

“I hope that one day I will rejoin a party that genuinely appeals to the many not the few, that can win again the kind of majority needed to improve the life chances of those who will be damaged by Brexit. In the meantime, please, for the sake of the party and especially for the sake of the country, think beyond the messenger, and think seriously about the message.”

Labour had no comment on Campbell’s letter. It comes after Peter Mandelson, another former Labour strategist under Blair, gave an interview over the weekend calling for a new party leader, although he said he would not leave the party.

“We are now in a situation where many people will react with horror against what Johnson plans to do – defying parliament to take Britain out of the EU without a deal to safeguard our trade and jobs – but have no faith in Corbyn’s desire or ability to block this,” Mandelson told Alain Elkann for La Stampa and other European newspapers.

“Corbyn is not the leader that Labour needs at this time. He cannot deliver. Even his supporters are realising this. We need an alternative who can stand up to Johnson and lead the country against the disaster which is in the making.”