The former chancellor, 65, will meet Tory MPs this morning and confirm his intention to have a third shot at the political job which has eluded him since he lost to William Hague in 1997.
Mr Clarke said last night: "The Conservative Party needs a popular leader who can broaden the appeal of the party and earn the respect of all sections of British society ... I will put the Conservative Party on track for Government."
He will start the first of a series of meetings with MPs and will make a major speech, one of several on major policy themes, tomorrow. The former chancellor unveiled his leadership bid to the Daily Mail and launched a website: www.kenclarke.co.uk.
Mr Clarke told the Daily Mail: "The political health of Britain has deteriorated very sharply. The Conservative Party must do something about it. And I am the man to do it."
He added: "I am determined that Britain should be governed better than it has been under New Labour. I am horrified by a Government run on a basis of spin."
Mr Clarke's announcement came after weeks of speculation from supporters and opponents, and hints from the jazz-loving, cigar-smoking MP, who said in a weekend interview: "I want to give this a bloody good go. The only job I want is Prime Minister."
The entry of such a flamboyant figure from the left of the party will ignite the phoney war to succeed Michael Howard. It will place pressure on his leading rivals, the shadow Education Secretary, David Cameron, and the shadow Home Secretary, David Davis, who until now has been regarded as the front-runner.
Mr Clarke, whose strong Europhile views have made him unacceptable to many Eurosceptic Tories in the past, signalled his continued drive for the leadership earlier this month when he used an interview to admit he had been wrong about the success of the euro and insisted that Britain was at least a decade away from joining.
He is likely to sell himself to his party as the only contender who can score significant points against Tony Blair and, more crucially, the Prime Minister's likely successor, Gordon Brown.
He told this weekend's Sunday Times that the Liberal Democrats were " terrified" of him, and added: "I find it almost comical the number of people who tell me they would vote Tory if I were leader."
Opponents will highlight his Europhile record and his age, after his contemporary Michael Howard declared himself too old to lead the party into the next election.
The bookies cut the odds yesterday on Mr Clarke's chances. He has assembled backers from throughout the party, including Ann Widdecombe and Tim Yeo, who abandoned his own leadership ambitions at the weekend to back Mr Clarke. Mr Yeo told Channel 4 News: "For the first time perhaps since Margaret Thatcher we will have at the head of the Conservative Party someone who is genuinely an equal match for Tony Blair." Another Clarke ally said: "We're confident and absolutely convinced we have a broad-based support."
Mr Clarke's rivals greeted his entry into the race with not a little disdain. Sir Malcolm Rifkind, the former foreign secretary, said: "If you have a leader whose views on something as fundamental as the European Union are different to the rest of the party then that will only end in tears."
One Cameron ally said: "Most MPs will feel that someone who will be 70 at the next election is too old to lead at the next election."
Twice a loser
Mr Clarke was one of the first senior Conservatives to throw his hat into the leadership ring after Labour's landslide swept the Conservatives out of power. But under the then Conservative leadership rules, he lost to William Hague in a ballot of MPs.
Under revised leadership rules, giving the party rank and file the final choice of leader, Mr Clarke emerged as the contender to the backbencher Iain Duncan Smith after a series of ballots of MPs, but he was defeated in a ballot of the party membership.Reuse content