Ken Clarke has said he would run for the Conservative leadership if he thought he could win and the party was ready to change to broaden its appeal.
Mr Clarke has twice failed in Conservative leadership contests
He was still interested in becoming prime minister - "the one big office that matters in politics" - Mr Clarke told ITV's Jonathan Dimbleby show.
The party had to figure out how to change to win over Labour and Liberal Democrat voters, he added.
The 64-year-old ex-chancellor also said he was not too old to become PM.
"I have a habit of running for the leadership, which means I think my friends know perfectly well I would like to be prime minister," Mr Clarke said.
He said that whether he challenged for the leadership would depend on persuading "enough people who I want to persuade to help me get there".
"There isn't going to be a contest until the autumn, so I have got another couple of months at least in which I can weigh up, firstly, do I have a reasonably good prospect of winning and, secondly, do I have a reasonably good prospect of winning with a party that will change in the way I would like to see it change."
Spending more than 10 years in opposition was "ridiculous", Mr Clarke said.
"The Conservative Party have got to ask themselves, 'How do we persuade people who at the moment are voting Labour and Liberal Democrat to vote Conservative?'"
He also told the programme: "I don't think at my age... you can start ruling people out in politics - when I am 78 I shall probably be canvassing for pope.
"I have certainly not got re-elected to retire, and I shall certainly start trying to push my influence in politics as far as I possibly can."
His leadership ambitions suffered a double blow this weekend when Esher and Walton MP Ian Taylor, who helped run Mr Clarke's two previous challenges, and former education spokesman Damian Green gave backing to rival David Davis.
"David Davis could be the man for me," Mr Taylor said.
"David has a strong base within the party on the right wing and that is the more dominant part of the party."
Mr Green said: "To elect a leader who was brought up on a tough council estate in south London will send a very clear signal the party is about everyone in this country, not just a privileged few."
But Skipton and Ripon MP David Curry told the Daily Mail newspaper: "You have to ask who would be the best person to take on [Gordon] Brown in a real political maul and score points. As far as I am concerned there is only one man and that is Ken."
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Mr Clarke said: "[Mr Brown] shadowed me for about four years, I think. He is better than he was in those days - he is quite formidable but I enjoy debating with him.
"I also think he is a very vulnerable chancellor at the moment... we could be more prosperous really and I think the present state of the economy is rather fragile."
Shadow transport secretary Alan Duncan said on Sunday there was "massive feeling" among Conservative MPs that the leadership contest should happen soon, rather than towards the end of the year.
"If it could be done in a dignified, sensible and clear way", it should take place before the summer recess, he told Sky News' Sunday with Adam Boulton programme.
Mr Duncan described Mr Clarke as "a bit of a latecomer", and said: "It is clearly mainly now a fight between David Davis and David Cameron."