The "coup" against Labour leader Jeremy Corbyn "has failed" and trade union leaders can now broker peace within the party, Unite leader Len McCluskey says.
He called on MPs to "desist" from any formal leadership challenge to Mr Corbyn - who he said had been the victim of a "political lynching".
Labour MPs opposing him had been "seduced by sinister forces", he added.
Writing in the Sunday Mirror, Mr Corbyn said he was "ready to reach out" to Labour MPs who oppose his leadership.
Appearing on the Andrew Marr Show, the general secretary of Unite said trade unions were "professional negotiators" and could "resolve this issue" inside the Labour Party.
"The trade unions can broker a peace - with Jeremy as our leader and the genuine concerns of the PLP, we can bring people together," he said.
It follows dozens of resignations from the Labour front-bench team and a motion of no confidence in Mr Corbyn, passed by 172 to 40 Labour MPs on Thursday.
Former party leaders - including Lord Kinnock - have also called on him to resign.
'Man of steel'
However, Mr McCluskey said it was "unhelpful" for ex-Labour leaders who had lost previous elections to be "dragged out to be part of an unedifying coup".
He said Mr Corbyn had been "undermined, humiliated and attacked", but said he was "made of stronger stuff".
"He is a man of steel and he's made it clear he will not step down," the Unite boss added.
Mr McCluskey said legal opinion was "crystal clear" that Mr Corbyn would definitely appear on the ballot paper of any future leadership election.
However, Lord Kinnock - who faced a leadership challenge from Tony Benn in 1988 - told the same programme the Labour leader would have to secure nominations from MPs.
He would require support by 20% of MPs and MEPs, which meant there was "no basis on which Jeremy really could or should stay", he added.
Lord Kinnock repeated his call for Mr Corbyn to stand down - saying there had been "a significant shift" away from supporting him at a grassroots level.
He said there were now "deep residual doubts" about his ability to lead the party to electoral success.
Despite speculation, no Labour MP has yet confirmed they will stand against Mr Corbyn in a leadership contest.
However, a number of MPs have called on him to resign, in the wake of the EU referendum.
Former prime minister Tony Blair told Sky's Murnaghan programme he had "resolutely refused" to intervene in the debate about Mr Corbyn's leadership.
But he said: "For a democracy to function, you've got to have an opposition with the minimum level of credibility to challenge the government and to hold it to account."
Labour MP Chris Bryant, who resigned from the shadow cabinet last week, said the present situation was "unsustainable and the only person who can break that log-jam is Jeremy".
Mr Corbyn's leadership could "break the back of the Labour Party", he added.
Barry Gardiner, a shadow cabinet member and Corbyn ally, said someone who has "stayed neutral" could bring the two opposing sides together, suggesting Lord Prescott.
The former deputy prime minister did not rule out performing such a role, but he told BBC's the Daily Politics a leadership election could leave Labour "bitterly divided".
Lord Prescott - who said he did not vote for Mr Corbyn - argued that while the Labour leader had to improve, MPs also had to recognise "the course the party is on" and come together to "avoid a civil war".
Shadow chancellor John McDonnell told Murnaghan there had been a "mass hysteria" sweeping through Westminster following the EU referendum, but called on Labour MPs to "keep calm".
Emily Thornberry, a close ally of Mr Corbyn, urged MPs to "take a step back".
The shadow foreign secretary told ITV's Peston on Sunday: "I think the future of the country is at stake here."
Writing in the Sunday Mirror, Mr Corbyn said: "I am ready to reach out to Labour MPs who didn't accept my election and oppose my leadership - and work with the whole party to provide the alternative the country needs.
"But they also need to respect the democracy of our party and the views of Labour's membership, which has increased by more than 60,000 in the past week alone.
"Our priority must be to mobilise this incredible force to oppose the Tories, and ensure people in Britain have a real political alternative.
"That is my priority and always will be as leader of our party.
"Those who want to challenge my leadership are free to do so in a democratic contest, in which I will be a candidate."