Cannabis use in medicine is currently banned, but many doctors - including the British Medical Association - believe the law should be changed.

The farm will established at a secret location in the south-east of England.

Owner GW Pharmaceuticals was given security advice by the Home Office and Special Branch before setting up the facility.

High fence

A high perimeter fence surrounds the £4m greenhouse complex, and anyone going in and out is exhaustively checked.

The Home Office has granted two specific licenses, one to cultivate cannabis plants and a second to store and dispense the cannabis preparations for research.

Could medical research add to pressure for legalisation?

A Home Office spokeswoman said five other licences had been granted for medical research into cannabis.

"This licence is purely for research," she said, "But if and when the medical benefits of cannabis based products can be clearly demonstrated the government would be willing to propose amendments to drug legislation."

Worth huge sums

The farm's location is being kept secret because the plants being cultivated would be worth huge sums on the black market.

Initially the complex will develop standardised extracts of cannabis plants grown under controlled conditions.

Research will also be concentrated on the best way to give the drug to patients, and on which conditions it can safely treat.

Establish the facts

GW Pharmaceuticals founder Dr Geoffrey Guy has 18 years' experience in drug development.

He said: "There is a considerable body of evidence to suggest that cannabis may have a number of medicinal uses ... but there have been very few systematic research programmes or controlled clinical trials. Our aim will be to establish the medical facts."

Cannabis is chiefly seen as a pain reliever and a means to prevent nausea. It may be useful for the relief of pain and spasticity in multiple sclerosis, and for relieving pain in other neurological disorders, such as paralplegia and neuralgia.

There is evidence that it could also stimulate the appetites of Aids patients with wasting disease, and prevent nausea and vomiting associated with cancer chemotherapy. It may also be helpful in the treatment of the eye disease, glaucoma.