The warning comes on the day that Home Secretary David Blunkett announced that cannabis is to be downgraded from a class B drug to class C. This means that possession would lead to a caution, rather than arrest.

Many young people are simply not aware that smoking cannabis may put them at increased risk of respiratory cancers and infections

Dame Helena Shovelton

The British Lung Foundation is currently carrying out a review of research into the impact of smoking cannabis on health.

Preliminary results suggest that the drug is at least as harmful as smoking tobacco - and may carry a higher risk of some respiratory cancers.

Ignorance

BLF chief executive Dame Helena Shovelton said: "Many young people are simply not aware that smoking cannabis may put them at increased risk of respiratory cancers and infections.

"The government spends millions of pounds a year on smoking cessation and public education about the dangers of smoking, yet smoking cannabis is at least as harmful as smoking tobacco and, indeed, may carry a higher risk of some respiratory cancers."

She said regardless of cannabis's legal status, many young people will make their own decision about whether they wish to use it or not.

"The government have a public health responsibility to ensure they do so with full knowledge of the risks associated with smoking cannabis," she said.

"Fifty years ago, people thought smoking was a good thing. Now it is described as a public health disaster - we don't want to see the same thing happen with cannabis."

Under estimate

The report is also expected to warn that research carried out in the 1960s and 1970s may underestimate the impact of today's cannabis since it is much more potent than the cannabis smoked then.

The average cannabis cigarette smoked in the 1960s contained about 10mg of D9-tetrahydrocannabinol (THC), the ingredient which accounts for the psychoactive properties of cannabis, compared to 150mg of THC today.

British Lung Foundation chairman Dr Mark Britton said: "There is an urgent need for more research into the effects of cannabis on respiratory health so that we can feel confident in the advice we give to young people.

"As a respiratory consultant physician, I see the devastating consequences of smoking-related lung diseases, such as emphysema and chronic obstructive airways diseases, every day and I am fearful that long-term cannabis smoking may be linked to similar conditions."