Tony Blair has endorsed Ed Miliband's tactics for confronting the trade unions, saying he wishes he had done the same when he was Prime Minister.
Blair had previously gone out of his way to avoid commenting on Miliband's battles with Unite in the aftermath of the Falkirk vote-rigging scandal.
But on Tuesday morning, Blair - appearing on camera in front of a photo of himself - told Sky News: "I think this is a defining moment.
"It's bold and it's strong. It's real leadership, this.
"I think it's important not only in its own terms, because he's carrying through a process of reform in the Labour party that is long overdue and, frankly, probably I should have done it when I was leader.
"But at the same time what he's doing, and I think this is also very important for the country, is that he's sending a very strong message to the country that in the end he will do what's right, he'll govern for all the country and not simply for one section of it.
"This is big stuff and it takes a real act of leadership to do it."
Miliband is set to end the practice of automatically signing trade union members up to the party, in a move expected to place at risk £9m of funding.