The 39-year-old favourite in the Tory leadership race, who has been dogged by questions about whether he has used soft or hard drugs, also came under further pressure yesterday to clear the air about any personal experiences with drugs. David Davis, his nearest rival in the Tory race, raised the stakes by saying that he did not believe someone who had taken Class A drugs should become the party's leader or Prime Minister. "It's a breach of the law so if it was recent, the answer would be no," he told tonight's Morgan and Platell programme on Channel 4.
Mr Davis added that politicians should give a straight answer to straight questions. "The whole idea of drugs makes me nervous," he said. "Because, well, I just don't like the idea of taking drugs. I never have and I don't like it."
Mr Cameron has refused to confirm or deny that he used soft drugs as a student but said last weekend: "I did lots of things before I came into politics that I shouldn't have done."
Supporters accused his rivals in the leadership election of "dirty tricks" by encouraging the media to harry him over the issue. They claimed that one senior right-wing MP had said: "We are going to stuff him on drugs."
On Monday, Mr Cameron told sixth-form students: "I have seen people close to me have a very difficult time and wreck their lives through drugs." Yesterday Mr Cameron issued a statement after a newspaper reported that a member of his family was being treated for heroin addiction. The London Evening Standard reported that his relative was undergoing treatment at a clinic in South Africa after attempts at therapy and rehabilitation in Britain failed.
Mr Cameron said the individual had now been through rehabilitation and that he was "incredibly proud" of the way the person had come through their problems.
"Someone very close in my family has had a dreadful problem with drugs," he said. "They have come through it, been through rehabilitation, and I'm incredibly proud of them. Their life has nothing to do with my candidature for the leadership of the Conservative Party. Many families will have had a similar experience, and they and I know full well the damage drugs can do. I hope now that this person can be left alone. I won't comment further on this story."
The unlikely issue of drugs has moved to the centre of the Tory leadership election and was raised at two hustings meetings addressed by the four candidates this week. With the first ballot of Tory MPs taking place next Tuesday, senior Tories believe that any further controversy could undermine Mr Cameron's prospects. "It's the one thing that could cost him the prize," one said.
But the Cameron camp is optimistic that he will survive the storm, and win the backing of more than 40 MPs in Tuesday's ballot. The Davis camp claims the backing of 67, Kenneth Clarke 23 and Liam Fox 20.
The candidate who comes bottom on Tuesday will be eliminated. Another will go in the second round on Thursday. The two remaining contenders then go into a ballot of the party's 300,000 members.
Many Tory MPs believe the run-off will be between Mr Davis, the early front-runner, and Mr Cameron. But the Clarke and Fox camps insist the race is still wide open. There are growing fears among his supporters that Mr Clarke will fail in his third attempt to become Tory leader because he risks being eclipsed by Mr Cameron as the main standard-bearer of party moderates.
Yesterday the 65-year-old former chancellor mounted a direct attack on Mr Cameron's relative lack of experience. He said: "He hasn't been in national politics. He's been in the House of Commons for four years but he's never actually played any role in national politics."
Mr Clarke added: "One speech and a fortnight's publicity and he's up there as a prime minister candidate. There are quite a few weeks to go yet so people will almost certainly reflect on this coming from nowhere."
But Keith Hellawell, the Government's former drugs tsar, said Mr Cameron should come clean about his past. "It matters because now the public is interested in knowing whether this man was involved in drugs in the past," he said.
Yesterday Mr Davis promised that the Tories would be "back in business" in the cities if he wins the leadership. Speaking in Bradford, he said: "The modern Conservative Party doesn't exist to protect privilege or preside over the status quo. Our mission is to change Britain and improve lives - and we will begin in the cities, where the hope of change has been absent for too long."