Tony Blair says he now accepts Jeremy Corbyn could become prime minister.
The ex-PM told BBC Newsnight that a year ago he would have said it was impossible for the left-wing Labour leader to win.
But he added: “There’s been so many political upsets, it’s possible Jeremy Corbyn could become prime minister and Labour could win on that programme.”
Mr Blair, a consistent critic of Mr Corbyn, said he had not changed his mind on the “wisdom” of electing him.
Having defied predictions of a heavy defeat at last month’s general election – and stripped the Conservatives of their majority – Mr Corbyn now describes his party as a “government-in-waiting”.
Many of his critics have since admitted they underestimated him.
Speaking to Newsnight, Mr Blair said he still believed “it’s a surer route to power to fight from the centre” and that it would be damaging for the country if Mr Corbyn became prime minister and imposed “an unreconstructed far Left programme”.
But on Mr Corbyn’s chances of reaching Downing Street, he said nothing could be ruled out.
“For most of my political life I’ve been saying: ‘I think this is the right way to go, and what’s more it’s the only way to win an election’.
“I have to qualify that now. I have to say ‘no – I think it’s possible you end up with Jeremy Corbyn as prime minister.'”
The Labour leadership has dismissed Mr Blair’s recent interventions – which included claiming Brexit followed by a Corbyn government would leave Britain “flat on its back”.
“To be frank, Mr Blair hasn’t really listened to the nature of the debate that is going on in the pubs, the clubs and school gates etc,” shadow chancellor John McDonnell said on Saturday.
The interview will be shown on Newsnight, on BBC Two, at 22:30 BST on 17 July.
Tony Blair: I think Corbyn could be PM}