David Miliband has thrown himself into Labour's by-election infighting by declaring the party is at its weakest for 50 years, while also refusing to rule out a comeback.
The former foreign secretary and Blairite standard bearer says Labour is now weaker than it was in the 1980s and must confront the "historic nature" of the challenge it faces.
Mr Miliband was speaking in an interview in The Times after Labour's defeat by the Conservatives in the Copeland by-election, which has prompted calls from some Labour MPs for Jeremy Corbyn to quit.
Labour's defeat in Copeland, which it had held for 80 years, was the first for an Opposition party by a Government party since the Tories defeated a Labour defector to the SDP in 1982.
Mr Miliband, who was narrowly defeated by his brother Ed in the 2010 Labour leadership election, is still seen as a leader in exile and potential saviour of the party by some MPs still loyal to Tony Blair.
In his Times interview, Mr Miliband, 51, said: "I'm obviously deeply concerned that Labour is further from power than at any stage in my lifetime."
And asked about his own future and a possible comeback, the former South Shields MP who quit to become a New York-based charity boss in 2013, said: "I honestly don't know what I'm going to do.
"It's hard to see, but what is the point of saying never."
His tantalising refusal to rule out a dramatic return to UK politics will delight Blairite MPs demoralised by Labour's lurch to the left and slump in electoral support under Mr Corbyn and his allies.
Mr Miliband said the rise of Emmanuel Macron - who met Theresa May in Downing Street this week - in the French presidential race showed there was still a market for a pro-European centrist approach.
"The Macron phenomenon is real," he said.
"He has built something extraordinary. I don't think the centre left is ever on the way out globally, because it has got a lot to say.
"As long as there are people with the values who yearn for a different kind of society then you're going to have a party that tries to put those values into practice."
Mr Miliband said, however, that it would be a mistake for Mr Corbyn's critics in the Labour Party to follow Mr Macron's example and found a new party, as the Gang of Four did with the SDP in the 1980s.
After the Copeland defeat, Mr Corbyn is being blamed for a by-election disaster and is facing renewed calls to resign before the 2020 general election.
This weekend he attends the party's conference in Scotland, where Labour has just one MP out of 59.
But he has received strong backing from Scottish Labour's leader, Kezia Dugdale, who told Sky News the leadership issue had been settled last year.
Speaking at the conference in Perth, Labour deputy leader Tom Watson will give his backing to Ms Dugdale's call for a federal system of government in the UK, calling it "power to the people".
Mr Watson will say: "In this post-Brexit world I favour devolution all round. I favour the regions and nations taking from Brussels powers over agriculture and fisheries, regional policy, social funds, environmental protection.
"And sharing control of the £4bn-worth of spending by Europe in the UK."
He will also say: "And we should go much further than this. We can unite together on a radical programme of constitutional change for the regions and nations of the UK.
"Not everyone forced to go at the same pace; that would be against the spirit of local control of decision making. But everyone benefiting from less centralisation and less London control and benefiting from more devolution and more power to the people."