Young men who smoke marijuana are more likely to develop an aggressive form of testicular cancer than those who have never tried the drug, a study has found.

Smoking the drug at least once a week, or using it regularly from adolescence, doubled the risk of a fast-growing form of the disease called nonseminoma, which tends to strike men in their 20s and 30s, researchers said.

The US study is the first to find evidence of a link between cannabis and testicular cancer, which is the most common type of cancer among British men aged 20 to 44. More than 1,900 new cases of the disease are diagnosed in the UK each year, but it responds well to treatment, with nine in 10 men surviving.

The findings suggest that smoking the drug before the age of 18 raises the cancer risk by coaxing immature cells in the testes to become tumours later in life.

Scientists at the Fred Hutchinson Cancer Research Centre in Seattle investigated the possibility of a link after learning that the testes were one of the few organs in the body to contain receptors for the main psychoactive substance in the drug, tetrahydrocannabinol (THC). There has also been a rise in testicular cancer cases that has mirrored the rise in marijuana use since the 1950s, they said.

"Our study is not the first to suggest that some aspect of a man's lifestyle or environment is a risk factor for testicular cancer, but it is the first that has looked at marijuana use," said Stephen Schwartz, an epidemiologist and author on the study.

The researchers asked 369 testicular cancer patients if they had any history of marijuana use. A further 979 healthy men were asked about their use of the drug.

After accounting for any family history of the cancer and lifestyle factors, such as smoking and drinking alcohol, the study found cannabis use emerged as a significant, separate risk factor for the disease.

Being an existing cannabis user raised the risk of cancer by 70%, while men who had used the drug regularly from puberty were twice as likely to develop the disease than those who had not used the drug.

Men naturally produce a cannabinoid-like substance that is thought to protect the testes against tumours. But smoking cannabis may disrupt this and so raise the risk of cancer, the study speculates.

Ecstasy remains class A

The home secretary will reject calls to downgrade ecstasy to a class B drug this week in a move that risks igniting a fresh row with its own drug advisers. Jacqui Smith will be urged to remove ecstasy from the class A category, which it shares with heroin, in a report to be published on Wednesday by the government's Advisory Council on the Misuse of Drugs. The report follows a year-long ACMD review of the health risks associated with ecstasy after a request by the all-party science and technology committee. The Home Office has made it clear it has no intention of reclassifying the drug. In January, the government upgraded cannabis to class B, against ACMD advice.