Tony Blair was branded "stupid" by one of his closest cabinet allies to have announced two years ago that he would not fight the next election.
John Reid warned that the Prime Minister's imminent departure, as well as party infighting, could have a devastating impact on Labour's electoral appeal.
The Home Secretary unmistakably set out his leadership stall yesterday as new research showed him as the clear choice of voters for the next Prime Minister.
In a blow for the Chancellor, just three of 30 voters assembled for a BBC2 Newsnight focus group chose him as the best successor to Mr Blair compared with 17 who singled out Mr Reid.
The Home Secretary told a fringe meeting that he believed Mr Blair had seriously blundered in 2004 by pre-announcing his eventual resignation a leader. He said: "Far from helping Labour win a victory at the next election, Tony Blair's loss will be a disadvantage to us ... He was stupid to himself and to our prospects for saying he was going to go, but he said it."
Mr Reid, who has so far refused to be drawn on whether he will make a pitch for the leadership, said the "disunity and introspection" of recent weeks had been a disaster for the party. He added: "It looked to the public like an introspective, self-interested squabble."
The Newsnight focus group was shown biographies, speeches and interviews by six potential leadership candidates, including Mr Reid, Mr Brown and the left-winger John McDonnell. Asked who they would like to see as next Labour leader, 17 said Mr Reid, three Mr Brown and three Mr McDonnell.
The same exercise during last year's Tory conference gave David Cameron a massive lead, and was seen as a turning point in that leadership contest.Reuse content