Three cannabis joints a day cause the same damage to the lining of the airways as 20 cigarettes, the British Lung Foundation study says.

When cannabis and tobacco are smoked together, the effects are dramatically worsened, it adds.

Evidence shows that tar from cannabis cigarettes contains 50% more carcinogens (cancer producing agents) than tobacco, it says.

Dr Mark Britton, chairman of the foundation, said: ``These statistics will come as a surprise to many people, especially those who choose to smoke cannabis rather than tobacco in the belief it is `safer` for them.

``It is vital that people are fully aware of the dangers so they can make an educated decision and know the damage they may be causing. The report is not about the moral rights and wrongs of cannabis, but simply making sure everyone is completely clear about the respiratory health risks involved.``

The report, A Smoking Gun?, also shows that the health dangers of cannabis have substantially increased since the 1960s, because of increased amounts of THC (tetrahydrocanabinol), the ingredient which accounts for the psychoactive properties of cannabis, in the substance consumed today.

The study looks at existing scientific and medical research on cannabis smoking and respiratory health.

Dame Helena Shovelton, chief executive of the foundation, said: ``Puff and inhalation volume with cannabis is up to four times higher than with tobacco - in other words you inhale deeper and hold your breath with the smoke for longer before exhaling. This results in more poisonous carbon monoxide and tar entering into the lungs.``