Admitting a youthful dalliance with soft drugs has almost become a rite-of-passage for Britain's politicians in recent years.

Everyone from Conservative chairman Francis Maude to home secretary Charles Clarke has owned up to puffing the occasional joint in their youth.

Provided the "experimentation" was brief, firmly in the past and subsequently regretted, the story is swiftly forgotten.

Yet - as would-be Tory leader David Cameron is finding out - the drugs issue still has the power to land politicians in hot water with the media and party colleagues.

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