Boris Johnson has given David Cameron major political headache over his admission that he had taken cocaine and cannabis as a teenager.

The Tory candidate for London Mayor confessed in an interview he had used cocaine when he was 19 and studying at Oxford.

He also said he had taken "dope" before that.

His admissions, which he has alluded to before but never fully confirmed, will open up Mr Cameron to fresh questions about his own drug use.

Scroll down for more...

The Tory leader, who was at Oxford with Mr Johnson, said in 2004 he had had the "normal student experience".

He has also failed to deny claims in a biography he had come close to being expelled for using cannabis at Eton.

Details of Mr Johnson's drug past emerged today in a new Marie Claire magazine interview by former TV presenter Janet Street-Porter.

When he was asked about his previous jokey references to cocaine use, he replied: "Well, that was when I was 19. It all goes to show that sometimes it's better not to say anything.

"I thoroughly disagree with drugs. I don't want my kids having drugs."

The Henley MP was referring to an appearance in 2005 on TV quiz Have I Got News For You in which he said: "I think I was once given cocaine but I sneezed and so it did not go up my nose.

"In fact, I may have been doing icing sugar."

Asked by Street-Porter whether he had smoked "dope" before university, Mr Johnson replied: "That's true but the stuff you and I may have smoked is not the same as what the kids are having now.

"I think skunk and this stuff is very, very dangerous."

Prime Minister Gordon Brown's plans to reclassify cannabis received a setback this week when an independent advisory committee said there were no grounds for tightening the law.

Former Home Secretary David Blunkett downgraded the drug.

Virtually the entire Cabinet last year admitted they had smoked cannabis, after Home Secretary Jacqui Smith revealed she had experimented with it as a youngster.

Everyone from Hilary Benn to Lords leader Baroness Ashton confessed to having dabbled with dope.

The only minister who refused to comment at the time was Culture Secretary James Purnell.