Kenneth Clarke, the avuncular "big beast" of the Conservative Party, today ended months of speculation as he launched his third bid to become its leader.

Mr Clarke's announcement was swiftly followed by that of Sir Malcolm Rifkind, the shadow Work and Pensions Secretary, firing the starting gun for the leadership race proper after a phoney war which has raged since May.

Announcing his candidacy, Mr Clarke, 65, confronted head-on the accusation that he will be too old to be Prime Minister by the time the next general election is called in five years' time.

His opponents in the party ranks have been careful to remind their colleagues that Michael Howard, who resigned the leadership to make way for a fresh face, is a year younger. "In this day and age it’s quite ridiculous to say I’m too old," Mr Clarke, famously fond of jazz and cigars, said.

"Actually, if you look around politics there are some very active politicians who are a lot older than I am," he added.

Mr Clarke will spend today with supporters before making a major speech in London tomorrow, laying out the blueprint for his campaign.

Sir Malcolm's entry into the race has surprised some Westminster watchers. Although he had clearly signalled his intention to run, many expected that he would use Mr Clarke's announcement today as a convenient opportunity to withdraw.

Instead, the two members of the Tory old guard, who served together in power under Margaret Thatcher and John Major are now political rivals, although both subscribe to the notion of One Nation Conservatism. On past form, Mr Clarke has the edge, having held the posts of home secretary and chancellor, while Sir Malcolm was defence and then foreign secretary.

Sir Malcolm, 59, who was parachuted in to the ultra-safe Kensington & Chelsea seat in the May election after failing to recapture Edinburgh Pentlands from Labour in the 2001 election, dismissed suggestions that his early entry into the race was a deft manoeuvre to scupper Mr Davis and ensure a plum job in the shadow Cabinet.

"I am a serious candidate, I will be a candidate," he told Sky News.

While Mr Clarke proclaimed that he had the credentials to be "popular leader" both within Tory ranks and the wider public. But despite renouncing his vocal support for the euro and proclamation that the EU Constitution is "dead", his Europhile leanings could still alienate him from much of the party. He has, however, already obtained the backing of Ann Widdecombe, Tim Yeo, John Bercow, Andrew Tyrie and David Curry.

Sir Malcolm's bid is being seen as an attempt to steer a course between the liberal Mr Clarke and David Davis, the Shadow Home Secretary, an early favourite despite having yet officially to throw his hat into the ring.

David Cameron, the Shadow Education Secretary, is also expected to announce his bid for leadership in coming weeks and is Liam Fox, the Shadow Foreign Secretary who is also regarded as a fringe candidate.

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