Tony Blair's warning that a "heavyweight" would knock the Conservatives out at the next election was a reference to Gordon Brown, Hazel Blears has said.
The Labour party chairwoman said the prime minister "was talking about the chancellor of the exchequer" - although Mr Blair himself has refused to confirm this.
Ms Blears told BBC Two's Daily Politics he 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".
The prime minister has refused to endorse the chancellor as the next Labour leader since the failed coup against him, which Mr Brown was suspected of orchestrating.
But during a heated debate with Mr Cameron, the Tory leader, after the Queen's speech yesterday, Mr Blair said: "The next election will be a flyweight versus a heavyweight.
"However much the right honourable gentleman may dance around the ring beforehand, at some point, he will come within the reach of a big clunking fist.
"And you know what, he will be out on his feet, carried out of the ring - the fifth Tory leader to be carried out, and a fourth term Labour government still standing."
The prime minister was rewarded with a grateful pat on the back by Mr Brown when he sat down after his speech.
Speaking later, work and pensions secretary John Hutton refused to speculate on whether Mr Blair's "heavyweight" comment represented a strong endorsement.
"David Cameron will meet a formidable challenge at the next general election that's for sure. Who we choose as a leader is a matter for the Labour party," he told Channel Four News.
He added: "No one is in any doubt at all that the chancellor is a person of tremendous stature and achievement, and would be a very formidable and heavyweight challenger."
During the House of Commons debate, Mr Cameron praised Mr Blair's abilities and said he was happy he was leaving Downing Street - indicating that Mr Brown would be a much easier opponent come election time.
"Many on that side of the House [Labour] are relieved he is going, but I can tell you they are not half as pleased as we are. I did check he was going before I applied for the job," he told MPs.
Mr Cameron also attacked the chancellor for having nothing new to offer beyond the current Labour leadership's obsession with headline-grabbing laws.
"Because if the problem with this government is short-term gimmicks, politically motivated laws, and failed centralisation, the chancellor isn't the solution - he's the biggest part of the problem," he declared.
Shadow immigration minister Damian Green continued in this vein on BBC Two's Newsnight, saying: "Everyone assumes Gordon Brown is going to take over so I assume that this speech is as much Gordon Brown as it is Tony Blair.
"And what I think is significant is that this is a tired Queen's speech - there is no inspiration, there is nothing new in it."