The campaign to legalise cannabis will be given a big boost next week: an expert report by the British Medical Association will say there is evidence that the drug can indeed have medical benefits.

The report will say that new medicines based on cannabis should be developed and it will spell out the legal changes needed to allow that to happen. Labour MPs who have been campaigning for the ban on cannabis to be lifted for medical use think it could be the breakthrough they have been waiting for.

The BMA report will say that some of the 400 chemical constituents of cannabis can help to control nausea, an important finding for cancer sufferers who are using the drug illegally to enable them to continue with chemotherapy. The drug also has had benefits for patients in pain relief, and there are claims that it can help to control compulsive movements and glaucoma.

The report is based on a re-examination of published research but its appearance under the name of the BMA will give the findings added weight. It will increase pressure on the Government following the launch of the Independent on Sunday's campaign for the decriminalisation of cannabis and the call by the Lord Chief Justice, Sir Thomas Bingham, for a debate on the issue.

A BMA source said: "We don't talk about recreational use because that was outside our remit, but we say that for personal use, it would be better to develop new drugs. We are not suggesting that patients should be allowed to roll joints and smoke them."

The BMA annual conference came down in favour of a policy of legalising cannabis use for medicinal purposes, but the report will take the campaign a step further by confirming that there is medical evidence that it can have benefits.

Paul Flynn, the Labour MP for Newport West, who has been leading the campaign in the Commons for cannabis to be legalised for medicinal use, was delighted by the news of the report. "It is a breakthrough, and it could provide a big boost to the campaign," he said.

The Home Office minister George Howarth told Mr Flynn in the Commons there was no medical evidence that cannabis provided medicinal benefits. The Home Secretary Jack Straw has maintained a tough line in resisting all pressure for cannabis to be legalised, but Frank Dobson, the Secretary of State for Health, has been more sympathetic.

Mr Dobson has indicated that if medical evidence could be found, he would have no objections to cannabis being legalised for medicinal use only. Mr Flynn intends to challenge the Home Office again with the BMA findings.

If the law is changed, new drugs will have to be created and tested before they are made available.