Labour MPs' bid to oust Jeremy Corbyn "has failed", one of the leader's closest allies has claimed.
Len McCluskey told ITV News that the trade unions can help broker a peace deal and reunite the party.
Corbyn has suffered dozens of resignations from his frontbench team and a motion of no confidence was overwhelmingly backed by Labour MPs.
"This coup has failed," the Unite leader said. "A decent man, who has been attacked and humiliated and embarrassed, is stronger and sterner stuff than people think - he's a man of steel.
"He has got no intention of stepping down."
He urged Angela Eagle and Owen Smith, who have been touted as potential challengers to Corbyn, to step back and avoid "plunging the Labour Party into a civil war".
Mr Corbyn has insisted he is "ready to reach out" to his critics in the party but warned he would stand for re-election if they tried to oust him.
Neil Kinnock, the party's former leader, earlier called on Corbyn to "do his duty and resign".
He told the BBC's Andrew Marr Show that the Tories will be in power "for decades" unless he quits.
Emily Thornberry, the shadow foreign secretary, called for "calm voices and cool heads".
She told Peston on Sunday that ex-deputy prime minister John Prescott could help solve the current impasse.
Tom Baldwin, Ed Miliband's former adviser, also told Robert Peston that it was "impossible" for Corbyn to continue as leader and backed Angela Eagle to take over.