I worked with Ken Clarke for over 20 years I have seen at first hand the remarkable qualities he has

Sir Malcolm Rifkind

Sir Malcolm had originally backed defeated candidate Michael Ancram - who has now joined the camp of Mr Clarke's rival, Iain Duncan Smith.

The decision came as Mr Clarke warned that an Iain Duncan Smith victory would see the Conservatives "lurch to the right", leaving them unable to win the next general election.

The decision

Sir Malcolm said the Tories were faced with a decision between "two able men" but that Mr Clarke would make the better leader.

"I worked with Ken Clarke for over 20 years. I have seen at first hand the remarkable qualities he has - he can be a difficult customer, I don't doubt that for a minute - but he has the qualities of leadership that the public enjoy and which [show] the Conservative Party as being a true alternative government," he said.

Mr Clarke: Has warned the "headbangers" to lay off

"I am satisfied that he is committed to the modernisation of our party and that he will ensure that we are seen again by the electorate as relevant, competent and attractive as an alternative government."

As for the deeply divisive issue of Europe, while Sir Malcolm's views differed sharply from those of the former chancellor, he fully agreed with him "that the party must spend far more time on health, education and the many other social and economic issues that will be the public's priority in the years to come".

He added: "Whoever is leader, there will be differences on European policy, as there is in the Labour Party."

Mr Clarke has already declared that the "headbangers" on both the pro and anti-European sides must not be allowed to wreck the party over the issue.

'Reforming agenda'

He believes public services must be put before the issue of the European single currency.

He says the 330,000 party members being balloted on the leadership face a choice between his "reforming agenda" and the "status quo" favoured by his rival.

But some senior party figures fear the deep divisions over Europe could tear the party apart, whoever wins.

There have been suggestions that Eurosceptic members are saying they will resign if Mr Clarke becomes leader, with some on the left threatening similar action should Mr Duncan Smith win.

On Sunday, Mr Duncan Smith insisted he was the "underdog" in the race for leader despite topping two separate polls of Tory constituency chairmen.

Iain Duncan Smith: Inists he is the underdog

A survey of 201 local constituency chairmen put the right-winger ahead of rival Ken Clarke, with Mr Duncan Smith gaining 40% of the support, and the pro-European former chancellor 36%.

According to the survey, commissioned by the Sunday Times, the contest remains wide open with 24% of the chairmen saying they are "undecided".

'Underdog'

But Mr Duncan Smith told the BBC's Breakfast with Frost programme on Sunday: "I still believe that I am the underdog."

Both candidates now face a summer of wooing the Tory rank and file ahead of the first ever postal vote of the party membership in a leadership contest.

However, both Mr Clarke and Mr Duncan Smith have agreed to a truce for the first two weeks of August to allow them to take a holiday.

Another survey, conducted by Mori for the Mail on Sunday, polled 100 constituency chairmen in Conservative strongholds where membership is concentrated.

The survey found 46% backing Mr Duncan Smith, against just 28% for Mr Clarke.