Former Tory Party chairman and current Oxford University chancellor Lord Patten has waded into the Conservative leadership race by advising David Cameron to pull out -- on the day the Witney MP was due to officially announce he is standing.

Mr Cameron was due to launch his campaign in London today standing as the self-styled "candidate for change".

But in an exclusive interview with the Oxford Mail Lord Patten said 38-year-old Mr Cameron's time would come, but for the time being he was too politically naive to become a successful party leader -- and ultimately Prime Minister.

And he suggested that should Mr Cameron emerge successfuly from the Tory ballot, he would follow William Hague and Ian Duncan Smith into the political wilderness.

Lord Patten said: "There seem to be two questions that the Conservative Party should, for a change, be asking itself when selecting a new leader.

"The first is who would Mr Blair and Mr Brown least like to see and the second is who would the electorate most like to see? There is no serious debate that the answer is the same if we want to be a credible opposition with a chance of forming a Government -- Ken Clarke."

The Tories gather for their annual conference in Blackpool this weekend, which is being billed as a showcase for the potential leaders.

Each candidate -- including Mr Clarke, Mr Cameron, David Davis, Liam Fox and Malcolm Rifkind -- will have a chance to persuade grassroots activists of their credentials.

Party members will have a say in who becomes the next leader with an announcement expected in December.

Mr Cameron, who is married with two children, has positioned himself as the man who personifies change.

Despite being an Old Etonian he is the only candidate not to have served in the Conservative Party of the Thatcher years and his brand of "modern, compassionate Conservatism" aims to win back the trust of the British people.

Lord Patten added: "I have the highest regard for David Cameron, he is clever, talented, charming and brave and he will one day be leader of the Conservative Party.

"But he would be best served if his time came later.

"In 1997 we chose a very young leader who had experience of the Cabinet -- and it didn't work.

"I would like the Conservative Party to make itself electable again.

"Since John Major we have chosen three leaders and none have turned into the Prime Minister so it's about time we elected one.

"Remember Ian Duncan Smith? It was not a success.

"If you had told me Ian Duncan Smith would become leader of the Conservative Party I would have said 'you must be joking'.

"My advice to David Cameron is to sit this one out...it would be a breath of fresh air to have Ken Clarke as leader.

"Ken is such an accomplished politician and someone who could knock Blair around in the Commons," he said.