Labour has been reduced under Jeremy Corbyn to a "fringe protest movement" that is not serious about regaining power, Tony Blair said in a damning analysis of the state of the party.
The former prime minister said the new leader's moral values were worth little if he avoided the "hard thinking" required to return to power - dubbing the current situation a "tragedy".
Mr Blair - who led Labour to three successive terms of office from 1997 - made clear his frustration in an article for the Spectator magazine defending his record.
"All wings of the Labour Party which support the notion of the Labour Party as a party aspiring to govern, rather than as a fringe protest movement, agree on the tragedy of the Labour Party's current position," he wrote - noting that they included those critical of Blairism.
"Many - especially in today's Labour Party - felt we lost our way in Government," he went on.
"I feel we found it. But I accept in the process we failed to convince enough people that the true progressives are always the modernisers, not because they discard principle but because they have the courage to adhere to it when confronted with reality."
He said: "Of course we made mistakes. But we were a radical reforming Government. And we tried to put the moral purpose of the Labour Party into practice, the only sort of morality worth very much.
"Right now we're in danger of not asking the right questions, never mind failing to get the right answers.
"All of it is about applying values with an open mind; not boasting of our values as a way of avoiding the hard thinking the changing world insists upon."
Despite the resistance of elements in the party, Labour from 1997 achieved "dramatic" results - notably "transformative" school reforms, record NHS satisfaction rates and falling crime, he said.
He insisted his decision - still by far the most divisive within the party - to join the US-led "war on terror" was "made not in defiance of progressive politics but in furtherance of them".
"But the essential position we occupied in British politics retained appeal, which is why we not only won a third term in 2005, we were never in any real danger of not getting it."
He concluded: "My essential argument - and indeed with my own party today - is that this approach is not now redundant with time. It is even more critical.
"The pace of change is not slowing; it is accelerating. We have to understand it and prepare for it.
"Infrastructure, housing, social exclusion - all these challenges require more modernising and less ideological thinking."