Ed Miliband has called on successor Jeremy Corbyn to quit as Labour leader, saying his position is “untenable”.
The former party leader said he had supported Mr Corbyn “all the way along” but urged the Opposition leader to now reflect on what is “the right thing for the country”.
Britain is facing its worst crisis since the Second World War, Mr Miliband said.
He told BBC Radio 4’s The World At One programme: “I have reluctantly reached the conclusion that his position is untenable.”
Mr Corbyn’s camp insisted he was going nowhere and issued a “put up or shut up” challenge to Labour’s MPs.
Mr Miliband’s intervention follows a similar appeal from former acting Labour leaders Harriet Harman and Margaret Beckett for Mr Corbyn to fall on his sword.
Prime Minister David Cameron also waded into Labour’s misery, telling him: “For heaven’s sake man, go.”
Suggesting that Mr Corbyn’s political values would be best served if he quit, Mr Miliband said: “I am not a plotter, I am somebody who cares deeply about my country, deeply about my party, deeply about the causes that Jeremy and I care about.
“I think the best thing on all of those criteria is that he stands down, painful though that might be for him and many of his supporters.”
Mr Miliband said that if he had been in the same position “I would have gone” and added that a lot of Mr Corbyn’s work could continue.
“It’s more likely to continue, I think, if there is a more peaceful transition than a civil war in our party,” he said.
He said people across the party had reached the view that “at this time of acute crisis for the country, Jeremy cannot rise to that challenge”.
Mr Miliband said: “The problem is that it is precisely because of the gravity of the national moment, precisely because we have got to shape this moment in a progressive way not a right-wing reactionary way, that we cannot have a party leader that 75% or more of the representatives in Parliament don’t have confidence in.
“That is an unsustainable position and that is not ideological, it is just a fact of life.
“I deeply respect Jeremy as a person and indeed as a politician for the causes he has fought for. My judgment is those causes are more likely to be served if he goes.”