Jeremy Corbyn is fit to hold the office of prime minister, Ed Miliband said, but he would not be drawn on whether he believes his successor as Labour leader will walk through the door of Number 10.
Mr Miliband said it is "a matter for the electorate" whether Mr Corbyn ultimately becomes prime minister.
His comments came after senior shadow cabinet minister Angela Eagle refused to say she believes Mr Corbyn and shadow chancellor John McDonnell are suited to high office.
On BBC Radio 5 Live's Pienaar's Politics on Sunday, Ms Eagle, the shadow business secretary, repeatedly sidestepped a question about whether she feels Mr Corbyn and Mr McDonnell are fit for the top jobs in British politics.
But former leader Mr Miliband, who quit after the party's general election defeat in May, said "of course" Mr Corbyn is fit to lead the country.
He added: "In the end that's a decision for the electorate, as I discovered to my cost."
Mr Miliband said there is a "strength in depth in terms of our membership" which Labour did not have before Mr Corbyn's leadership election victory.
"Jeremy Corbyn has doubled our membership," he told BBC Radio 4's Today programme.
"I have seen that myself in my own constituency. As a constituency MP - and I think this is quite important - I am seeking to work out how do we use these new members so that we can do what we didn't do fully under me, which is become a community organisation that actually is a presence in communities up and down this country."
Mr Miliband stressed he is "not going to be a back-seat driver", prompting a barbed comment from Today presenter Jim Naughtie who told him: "Having crashed the car it's difficult to do that."
The former leader said Mr Corbyn is "going to argue it in his own way".
"He has set out what he believes his mandate is for, which is anti-austerity, a different approach to foreign policy and participatory politics," he said.
Asked if he believes Mr Corbyn will become prime minister, Mr Miliband said: "That's a matter for the electorate. I'm not in the predictions game and, if you'll forgive me, after my experience at the general election, predictions aren't my thing."
The former leader's appearance on Today followed reports that he joked about the party's poor performance under Mr Corbyn.
The Mail on Sunday reported Mr Miliband told a group of MPs including Graham Stringer, who had been a critic of his leadership: "I bet you didn't think things would actually get worse."