Smoking three joints a day might do as much damage to the lungs as do 20 cigarettes, the British Lung Foundation says in a report today.

Tar from cannabis cigarettes has 50% more cancer causing chemicals than a cigarette, and smokers inhale deeper and hold their breath longer, the charity warns in a report on Britain's most popular illegal drug, says the charity.

It fears young people are unaware of the medical consequences of smoking the drug, far more powerful than in the 1960s due to a higher ratio of tetrahydro-cabinol (THC), the ingredient associated with its mind-affecting properties.

The foundation wants a government campaign to warn of the heightened risks of chest infections and cancers. It also wants more research into increased risk of nicotine addiction, and whether cannabis smoking leads to more emphysema and chronic bronchitis.

The report, A Smoking Gun?, comes as the government prepares to reclassify cannabis as a less harmful drug and endorse a more relaxed attitude to policing its use.

Mark Britton, the charity's chairman, believed it would surprise those who smoked cannabis rather than tobacco, in the belief it was safer. "The report is not about the moral rights and wrongs of cannabis, but making sure everyone is completely clear about the respiratory risks involved."

It concludes that there is strong evidence cannabis smoking increases the risk of lung cancer, which kills more than 34,000 Britons a year.

Benzopyrene, a constituent of tar in cannabis cigarettes, alters a gene that normally suppresses tumours. It accepts that evidence is less conclusive on chronic obstructive pulmonary disease, thought to kill another 32,000 a year.