His intervention came as Mr Cameron was forced to admit that a close relative had undergone rehabilitation for serious drug abuse.

The Cameron campaign was facing a nervous weekend before the first vote by MPs on Tuesday after days of persistent questions, which he has refused to answer, over whether he had taken drugs.

Mr Davis, who has despaired at the gathering momentum behind the Cameron campaign since the party conference, used a television interview to fire a warning shot to Conservative MPs to vote with care next week.

Asked on the Channel 4 programme Morgan and Platell whether Class A drug users could lead the Tory party or the country, he replied: “I think if . . . well, it is a breach of the law, so if it was recent, the answer would be no.” Mr Davis repeated that he had never taken drugs, before adding: “The whole idea of drugs makes me nervous.”

Asked whether politicians should gave “straight answers to straight questions”, he replied: “Yes.”

Mr Cameron yesterday issued a statement after a newspaper disclosed that a “close relative” had been treated for heroin addiction. The relative had been treated in a clinic in South Africa after attempts at rehabilitation in Britain had failed.

“Someone very close in my family has had a dreadful problem with drugs. They have been through it, been through rehabilitation, and I’m incredibly proud of them,” he said. He said the issue should have nothing to do with the Tory leadership contest, but the question is guaranteed to haunt him when he faces questions from MPs on Monday at the final hustings of the parliamentary contest.

Yesterday Mr Cameron had the support of Kenneth Clarke, another contender, who told local radio in his Nottingham constituency: “I would advise David to carry on refusing to answer questions of that sort.”

However, Mr Clarke could not resist exploiting his rival’s relative inexperience. “He’s been in the Commons for four years but he’s never played any role in national politics. One speech and he’s up there as a prime minister candidate. There are a few weeks to go yet, so people will certainly reflect on this coming from nowhere.”

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