Former Labour leader Ed Miliband has called for Jeremy Corbyn to resign from his post.
Mr Miliband told BBC Radio 4's World at One programme that though he had supported Mr Corbyn, he had "reluctantly reached the conclusion his position is untenable".
He said that he and others who wanted Mr Corbyn replaced were not "Blairites" but that they believed Labour needed to think about the country at a time of crisis.
"I’ve supported Jeremy Corbyn all the way along from the moment he was elected because I thought it was absolutely the right thing to do," he told the programme.
"I think we a lot of what he stands for is very important for us going forward but I have reluctant reached the conclusion that his position is untenable – because you are the leader of the Labour party in Parliament and in the country and it’s really important to understand that.
"I’m not a Blairite, at least I’ve never been called a Blairite, I’m not a plotter, I’m somebody who cares deeply about this country, deeply about my party, and deeply about the causes that I think Jeremy and I care about.
"I think the best thing on all of those criteria is that he stands down, painful though that will be for him and his supporters."
He added that a "peaceful" transition to another leader would allow some of Mr Corbyn's work and that of the shadow chancellor John McDonnell to continue.
Mr Miliband has remained on the backbenches since he stepped down as leader after the 2015 general election, declining to serve in Mr Corbyn's shadow cabinet.
He has however not before publicly criticised his successor.
Mr Miliband himself faced plots to oust him by the right wing of the Labour party during his time in office. He however saw off the threat of a coup.
Mr Corbyn has been hit by dozens of resignations from his shadow cabinet since the weekend, with his own MPs calling for him to quit. He still have substantial support amongst members of the party, who elected him by a landslide just in September however.
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