Drugs dominated the first day of head-to-head campaigning for the Tory party leadership today as David Davis took a public vow of silence on the issue and David Cameron insisted yet again that his past life was off-limits.

Mr Cameron, the 39-year-old Eton-educated moderniser who came from nowhere to become hot favourite to succeed Michael Howard, denied for the first time last night that he not taken Class A drugs during his four years as an MP.

But in a television interview he refused to discuss his experiences before then, ensuring that an issue that his team had hoped was dead and buried would once again make headlines.

Both candidates appeared on BBC Radio 4's Today programme this morning before a meeting with party leaders at Westminster to organise the six-week campaign to woo party members.

Mr Davis said previous comments he has made about drug policy have been "mischievously" interpreted as attempts to put pressure on his rival and insisted that he would not use the drug issue to try to gain an advantage over his rival.

"For the next six weeks, I am not going to answer any questions on drugs, policy or otherwise, because I am simply not going to have this debate dominated by this issue," he said.

Mr Cameron said: "I welcome what David has said, but I think I have made the right judgment, which is to say that you are, in politics, entitled to a past life that can be private and you don’t have to answer questions about that.

"Once you are in politics, people want to know who you are and what you are like and what you do and that you don’t break the law, and you must answer questions about those things. am very happy for people to look at me today and say ‘What sort of person are you?’. I’m a young guy with a young family and I lead a relatively straightforward life.

"People are already having a good poke around. I got up this morning and there were about six cameras and snappers outside my house and I’m happy about that. I expect they are looking in the rubbish bins."

The membership ballot will close on December 5, but experience suggests that roughly half of all members send in their ballots by return post once they are sent out in the first week of November, so the first three weeks will be crucial.

Officially, all party members should get the chance to mark their cross beside one of the candidates' names, but a grassroots group within the party warned party bosses that it would take legal action if there is any attempt to disenfranchise voters.

Reports this morning suggested that as many as 60,000 of the 300,000 members across Britain may not receive a ballot paper when they are sent out in a fortnight’s time.

Members who pay less than £15 a year to the party or have joined since 1998 may not get a vote, while husbands and wives who have joint membership may receive only one ballot paper, the BBC reported.

Christopher Montgomery, who led the Better Choice campaign that saw off Mr Howard's plan for his successor to be chosen solely by MPs, said that the number of ballots was "not going to be anything close" to the total membership and accused Francis Maude, the party chairman, of trying to boost Mr Cameron's chances.

"It’s an open secret that Francis and the outgoing leader would prefer David Cameron to win. They are the people who originally didn’t want the mass membership to get a vote," Mr Montgomery said.

"Now they are the people who are going to determine who among the mass membership gets a vote. There are moves afoot in Central Office to disenfranchise tens of thousands of Tory members. You don’t have to be too much of a cynic to say the people who are going to be disenfranchised might tend to be more at one end of the party than the other."

Mr Cameron will start his campaign with a visit to his constitutency in Witney, Oxfordshire, before a Tony Blair-style webcast this afternoon.

The 56-year-old Mr Davis is to underline both his humble background and what his supporters say is his broader geographical reach with a visit to his old university at Warwick.

Founded in 1965, Warwick is the country's most successful campus university, its alumni including Jennie Bond, the former BBC royal-watcher, and the pop star Sting.

It is still not clear whether Mr Cameron will find time over the next six weeks to visit his alma mater, Brasenose College, Oxford, founded in 1509 and named after a nose-shaped brass door knocker. Brasenose's alumni include William Webb Ellis, the creator of rugby football, comedian Michael Palin, and Lord Archer, the former Tory MP.

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