The chancellor was the "heavyweight" successor with "big clunking fists" referred to by Tony Blair, Labour's chairman Hazel Blears has told the BBC.
During the Queen's Speech exchanges on Wednesday Mr Blair told David Cameron that he was a "flyweight" who would be "floored" at the next election.
The prime minister has since refused to say whether the heavyweight he was referring to was Gordon Brown.
But Ms Blears said: "He was talking about the Chancellor of the Exchequer."
She told BBC Two's Daily Politics that Mr Blair had meant Mr Brown's "policies, particularly in relation to the economy, but also on other issues, could be a knockout blow for David Cameron".
Mr Blair himself refused to say who he was referring to, when responding to e-mailed questions during an interview broadcast on the Downing Street website.
He said: "I have decided to say nothing about it. You can say anything - people will always interpret these things but I have said all I want to say, at the moment."
And Mr Blair's official spokesman, asked separately about the "heavyweight's" identity, claimed the prime minister had been "speaking metaphorically".
'Delivered so little'
The boxing analogies in the Commons came as Mr Blair reacted on Wednesday afternoon to the Conservative leader's claims that the government was "tired and discredited" and peddling "the politics of fear".
Mr Cameron told MPs: "The tragedy of this prime minister is that he promised so much and he has delivered so little."
Mr Blair's 10th Queen's Speech featured legislation on climate change and pensions which will have their impact long after he has stepped down.
THE QUEEN'S SPEECH
Climate Change Bill
Serious Crime Bill
Criminal Justice Bill
Mental Health Bill
Road Transport Bill
Welfare Reform Bill
The package, which contained little that was not expected, was dominated by security measures to tackle crime, anti-social behaviour, illegal immigration and terrorism.
But Mr Blair's unexpected - and apparently unplanned - endorsement of a "heavyweight" successor has dominated the aftermath of the speech.
Amid a boisterous Commons, Mr Blair told Mr Cameron: "The next election, it will be a flyweight versus a heavyweight.
"And however much you may dance around the ring before times, at some point you'll come within the reach of a big clunking fist.
"You will be out on your feet, carried out of the ring - the fifth Tory leader to be carried out, and a fourth-term Labour government still standing."
Mr Blair has always declined to endorse Mr Brown as his successor when challenged to do so by Mr Cameron at prime minister's questions.
His words seemed to be the closest he has come to a public endorsement of Mr Brown, although, given the fact that there was no name uttered, some suggest he may have been referring to Home Secretary John Reid.
Mr Reid is widely seen as the most likely senior Blairite who might yet decide to challenge Mr Brown for the leadership when Tony Blair steps down.
Left-winger John McDonnell is the only person who has publicly declared that they intend to contend the vacancy, which is due to come by next August at the latest.
Anyone who wants to join the contest to succeed Mr Blair has to get the backing of 44 Labour MPs.
The candidates then go forward to a vote, with Labour MPs, Labour members and unions each making up one third of the electorate.