Ed Miliband has said Owen Smith is the Labour leadership candidate best placed to rise to the challenge of Brexit and unite the party.
Jeremy Corbyn has previously described Mr Miliband as a “great friend” and he was consistently rumoured to be joining the current leader’s shadow cabinet.
But Mr Miliband, who led the party before Mr Corbyn, said Mr Smith is the best candidate to make the crisis following the EU referendum “a progressive moment”.
In a video posted on YouTube, the Jewish MP said he was backing Mr Smith “for his vision, for his principles and for his ability to lead”.
Mr Miliband‘s comments come amid an intensifying battle for the Labour leadership and party organisation, with Mr Corbyn and deputy leader Tom Watson entering a new war of words.
Publicly, the pair’s relationship has remained relatively peaceful despite Mr Watson telling Mr Corbyn he should stand down after 172 MPs backed a no confidence motion in the leader.
But Mr Corbyn’s campaign team reacted furiously after Mr Watson accused “Trotsky entryists” of returning to the Labour Party and manipulating younger members to boost the leader’s chances of staying in post.
Mr Watson said “Trots” do not have the “best interests of the Labour Party at heart”, see it as a “vehicle for revolutionary socialism”, and are not “remotely interested in winning elections”.
The deputy leader insisted he does not believe that the vast majority of people who have joined the party who have been mobilised by the pro-Corbyn Momentum campaign group “are all Trots and Bolsheviks”.
He told the Guardian: “Some of these people are deeply interested in political change, in building a more equal society, and are just on a journey in politics that they’re new to, and I don’t want them to feel that I’m labelling them because I’m not.
“But there are some old hands twisting young arms in this process, and I’m under no illusions about what’s going on.
“They are caucusing and factionalising and putting pressure where they can, and that’s how Trotsky entryists operate.
“Sooner or later, that always end up in disaster.”
A Jeremy for Labour spokesman described Mr Watson’s claims as “baseless conspiracy theories” which patronise members.
And he accused the deputy leader of orchestrating the party’s appeal against a High Court ruling that new Labour members should be allowed to vote in the leadership election – an apparent boost to Mr Corbyn.
The spokesman said: “We regret that Tom Watson also forced through the decision yesterday at Labour’s NEC meeting to challenge the court judgment to restore the right to vote in the leadership election.
“Rather than patronising members and peddling baseless conspiracy theories about ‘Trotsky entryists’, he should be working with Jeremy to unite our party so that we can get back to campaigning to dislodge this Tory government, and help elect a Labour government in its place.”
Their exchanges followed the election of a slate of left-leaning candidates to Labour’s ruling National Executive Committee (NEC), seen by many as a win for Mr Corbyn.
One of the new members, Rhea Wolfson, appeared to threaten anti-Corbyn MPs with mandatory reselection to address the “important disconnect” between the parliamentary party and rank and file members.
She told BBC Radio 4’s World At One: “Ultimately we have to have a much more healthy conversation around reselection, if not mandatory reselection.”
But Andy Burnham said her comments were unhelpful and would only fuel a climate of distrust.
The Greater Manchester mayoral candidate told the programme: “To pull the rug from under our MPs or other elected representatives, I don’t think is helpful at this time, it fuels a climate of distrust and I don’t think that that is where we should be.”