A new political party "may be impossible" within the British system, Tony Blair has said as he urged "serious figures" in both Labour and the Conservatives to demand change of their leadership.
The former Labour prime minister between 1997 and 2007 said the forthcoming party conference season will also "expose in sharp relief the change state" of his former party and the Theresa May's Tories.
"They will provide evidence of the driving spirit inhabiting each main party, of the direction of the soul," he said.
His remarks follow months of speculation regarding the creation of a breakaway "centrist" party made up of those disillusioned with the leadership of Jeremy Corbyn.
But in an article for The Times newspaper, Mr Blair wrote: "I am not advocating a new party, organising one, or wanting to vote for one.
"In the British system such an endeavour may be impossible".
Mr Blair, a frequent critic of the Labour leadership, also used his article to suggest that under Mr Corbyn "outcasts from the political wilderness" have suddenly found themselves in governing positions within the party.
"With them has come their sectarian, shouty, finger jabbing, with a whole sub-culture of conspiracy theories - the left equivalent of the alt-right," he said.
The former prime minister also suggested that the Conservative government is "in a form of profound dysfunction" and is doing "deep disservice to the nation".
He added: "Yet neither the Labour Party nor its leader is ahead. As so often in history, attitudes towards the Jewish community are predictive of something larger. A world view has been laid bare and the nature of a large and growing party of today's Labour Party unmasked.
"Neither correspondent to the true Labour tradition," Mr Blair wrote.
Earlier this week, Mr Corbyn responded to similar criticisms from Mr Blair, telling the BBC: "I think Tony should recognise that the party membership is now much bigger than it's ever been. It's the biggest it's ever been in my lifetime.
"In the general election last year we set out what our aspirations are for the people of this country - sadly we didn't win, but we got the highest vote for Labour since 2001."
Jon Lansman, one of the founders of Momentum - the group set up to support Mr Corbyn's left-wing politics after his first leadership victory - added: "Tony Blair was never in the right party and there will never be a return to his politics in UK Labour."
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