Congress has previously taken a tough line on the issue, and voted by 310 to 93 against legalising it for medical use last autumn.

The director of Americans for Medical Rights, Bill Zimmerman, said the Institute of Medicine (IOM) report effectively said that "most of what the government has told us about marijuana is false".

Report author Dr John Benson: Three areas of clinical use

The report concluded that for some seriously-ill people, the benefits of cannabis outweigh its disadvantages.

Chuck Thomas of the Marijuana Policy Project said the report "shoots down" politicians' denials of cannabis' medical benefits.

The Clinton administration's anti-drug czar, Barry McCaffrey, has responded by calling for more research on cannabis - which has long been one of the US's most demonised drugs.

The IOM report suggests substitutes for smoking cannabis

"It's not addictive, it's not a gateway to heroin and cocaine, it has legitimate medical use, and it's not as dangerous as common drugs like Prozac and Viagra," he said.

Opponents of the medical use of cannabis say it starts people on the road to more dangerous drugs such as heroin and cocaine.

But the report said marijuana was not particularly addictive. It found "no conclusive evidence" that cannabis led to abuse of harder drugs.

Findings 'overdue'

Opponents argue that cannabis leads to harder drugs

Official tests on the drug have been opposed, but supporters of its medical use argue that it can relieve aches, pains, nausea and the loss of appetite associated with chemotherapy, multiple sclerosis and Aids. The drug is also said to help control glaucoma.

One long-term patient using cannabis said the findings were overdue.

"It's taken a long time, but I feel like now, people will stand up and listen," said Irvin Rosenfeld - a stockbroker who has smoked cannabis for 27 years.

He is one of just eight people in the country supplied with the drug by the federal government because of a rare medical condition.

Voters in six US states have chosen to legalise cannabis for severely-ill patients.

The first was California in 1996, but its decision was blocked by the federal government.

The issue has previously been raised by the New England Journal of Medicine, which has editorialised in favour of medical marijuana, and the American Medical Association, which has urged federal governments to support more research on the subject.

In 1998, a UK report supported research on medical marijuana

The IOM report - commissioned by the White House Office of National Drug Control Policy - comes on the back of a similar study in the UK by the House of Lords, which supported the medicinal use of cannabis.

British doctors are already testing the drug's potential for patients with multiple sclerosis and to relieve post-operative pain.

"What the Americans are saying now, is that there should be research into the therapeutic effects of cannabis. We are actually doing the research, so we are ahead of them," a spokeswoman for the Royal Pharmaceutical Society said.