Mon Nov 11,12:32 PM ET LONDON (Reuters) - Smoking three pure cannabis joints is as bad for your lungs as smoking 20 normal cigarettes and marijuana is more dangerous now than it was in the 1960s, British researchers said on Monday. In what it described as a shocking new report, the British Lung Foundation (BLF) said tar from cannabis cigarettes contained 50% more carcinogens--the agents that produce cancer--than tobacco. "Three cannabis joints a day cause the same damage to the lining of the airways as 20 cigarettes," it said in a statement. It also said the health dangers of cannabis have substantially increased since the 1960s because today's marijuana has increased amounts of a key chemical compound. Campaigners for the legalization of cannabis disputed some of the findings. "Saying that cannabis is stronger now than it used to be is like saying that orange juice is stronger these days," said Alun Buffry, spokesman for the Legalize Cannabis Alliance. "I smoked stuff in the 1960s which was certainly stronger than what's available now and, anyway, when it's stronger, people smoke less of it." BFL chairman Mark Britton 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," he said. The BFL stressed it was not taking a moral stance on the contentious issue of legalizing the drug. "We're not trying to say 'smoke' or 'don't smoke' cannabis," BLF Chief Executive Dame Helena Shovelton told BBC Radio. "We're saying that if you do, understand the risks involved in doing so. "Don't have the same situation we had with tobacco, which was years of denial about the problems," she said. The BFL urged the government to implement a public health education campaign on health risks of the drug.