In an open letter to Corbyn, Alastair Campbell said the party "no longer truly represents my values, or the hopes I have for Britain" and urged the veteran leftist Corbyn to consider standing down.

"With some sadness but absolute certainty, I have reached the conclusion that I no longer wish to stay in the party," he wrote in the letter published in The New European magazine.

Anti-Brexit crusader Campbell had begun appealing against his expulsion as a Labour member for voting for the Liberal Democrats in May's European elections.

But Campbell said Johnson becoming Conservative prime minister and adopting a "crash and burn strategy deliberately aimed at creating the circumstances for a general election" had altered his outlook.

"I see no sign that you and your office have grasped the seriousness of what is happening, let alone devised or begun to execute a strategy to respond and defeat it," Campbell wrote.

"Has the time not come for you to ask yourself, honestly, whether you have the capacity, the plan and the reach into the British public, to rise to the new challenge, and to win in the country?

"If you have doubt, then there is no doubt -- you should make way for someone who can provide the leadership, the strategy, and the energy required."

The ruling Conservatives have been boosted by a so-called "Boris bounce" since Johnson became Britain's new leader last Wednesday, according to weekend polls.

One showed a 10-point gain, putting the Tories on 30 percent, five points ahead of Labour.

Campbell, who served as Blair's chief press secretary and was heavily criticised for his involvement in drawing up Britain's Iraq War policy, emerged back into the political arena after the 2016 Brexit vote.

He has become a leading figure in the campaign to hold another referendum on European Union membership and has been a persistent critic of Corbyn's leadership, particularly over Brexit.