Labour's shift to the left under Jeremy Corbyn is a "mistake" that will not address the challenges Britain faces, says former South Shields MP David Miliband.
The former foreign secretary also said he would have rebelled against the Labour leader's decision to back the triggering of Article 50 to formally begin Brexit were he still an MP.
His comments come as Mr Corbyn resists fresh calls to resign following a humiliating by-election defeat at the hands of the Tories in the Cumbrian seat of Copeland.
Mr Miliband, who was narrowly beaten to the leadership by his brother Ed in 2010, called on Labour to "really understand the historic nature of the challenge" it now faces, with a recent opinion poll putting the party 18 points behind the Tories.
But he refused to join the calls for Mr Corbyn to stand down.
Mr Miliband told the Times: "I always say to people, 'Play the ball not the man.' So, 'What do I think of Jeremy Corbyn?' is not the question I ask myself. The question is, 'What do I think of the strategy?'
"The tempting thing to say is that it's a mistake because it won't get elected but for people like me it's a mistake because it won't address the challenges of the country.
"This isn't just an electability question, it's a question of substance. I think one can achieve more radical and substantive change through a different set of positions."
Mr Miliband also revealed he would have joined the 52 Labour MPs who rebelled against Mr Corbyn's three-line whip ordering MPs to back the European Union (Notification of Withdrawal) Bill.
He said: "I would have found it very difficult to vote for that because I think that Article 50 needed to be triggered but my fear is that the terms under which it's going to be triggered will be very damaging.
"I fear that we are going to end up with a higher chance of a disastrous outcome than is comfortable and I fear that we are going to end up with a harder outcome than is desirable."
Mr Miliband praised the "phenomenon" created by centre-left French presidential candidate Emmanuel Macron, who has formed his own political party, but cautioned moderate Labour MPs against following suit.
"There are very high barriers to entry both in terms of the emotional tug and the social and political roots of the Labour Party and in terms of the crowdedness of the political field," he said. "I'm not an advocate of that."
Mr Miliband, who is now based in the United States as the head of aid charity the International Rescue Committee, said Western governments' response to the global refugee crisis was a "threat".
And he criticised the Government for closing the Dubs scheme to welcome unaccompanied child refugees from Europe after 350 children were brought to Britain through it - far fewer than the 3,000 initially expected.
Commenting on the Government's position, he said: "It has been stout and strong and moral about international aid ... but on the other hand there was the decision not to allow the unaccompanied children in. I would never be ashamed of Britain but the position itself is shaming that we should decide not to honour the commitment to take in 3,000 kids."