Ex-Tory leader Sir Edward Heath says he thinks Michael Howard may be too old to win the next general election.
Sir Edward Heath was an MP for more than 50 years
Sir Edward, 87, said it would be "very difficult" for a man in his 60s to win over the younger generation which he says the Tories need to win power.
He told BBC One's Breakfast with Frost most recent prime ministers had been in their forties when elected.
"I think a whole lot of the population won't recognise somebody in their 60s as being a Prime Minister," he said.
Sir Edward, giving a rare interview, said he recognised Mr Howard's abilities, but admitted that he had never seen him as a potential leader of the party.
He predicted that at an election Labour and the Liberal Democrats would seize on Mr Howard's record as a minister under Margaret Thatcher and John Major.
But he did not say a Conservative election win was out of the question - saying that he thought the best hope of victory was the "confusion" in the ranks of the Labour Party.
Sir Edward was prime minister from 1970 to 1974, and was replaced as Tory leader by Mrs Thatcher.
Could Labour confusion help Howard
He has never been afraid to be critical of his successors as party leader, most frequently on the question of Europe.
Of Mr Howard, Sir Edward said: "The real point is that he is 62 or 63 and if the Parliament goes on for the full term, he will be 65.
"Today, the prime minister of the day is appointed in the forties. I think a whole lot of the population won't recognise somebody in their 60s as being a prime minister.
"It happened with Neville Chamberlain and Churchill in the war, but not in recent times, so he has got to overcome that as well.
"Our purpose should be to have somebody there that generations lower down - the young people - can say 'He will understand what we want and produce it for us'.
"It is very difficult to do that with somebody of a much greater age."
Of Tony Blair's government Sir Edward said history would judge that they wanted things to happen overnight, and they failed to see many things through.
Mrs Thatcher elected in 1979
He also said Tories' best election hope came from confusion within the Labour government: "We have got the chancellor of the exchequer and prime minister at loggerheads practically the whole time."
Sir Edward left Parliament in the 2001 election after more than half a century as an MP.
He insisted to Sir David Frost that "as a loyal Conservative", he admired all his successors as leader.
Even Lady Thatcher "is entitled to praise for the good things she did", he said. But pressed to name her greatest achievement, he declined to comment.