Professor Tony Moffatt describes the trial

They have requested money from the government's Medical Research Council to test both an extract of the plant, and one chemical which is thought to be the key ingredient.

Although cannabis is illegal in the UK, many MS sufferers are prepared to break the law because they say it offers effective pain relief.

This would be the first large-scale study using strict scientific critieria to measure how useful cannabis is for pain control.

'Proof is needed'

Professor Tony Moffatt chief scientist at the Royal Pharmaceutical Society, is supervising the bid for money from the government.

He said: "Because there seems to be good evidence out there, what we need to do is prove it scientifically.

"There have been a number of clinical trials but they have been too small or their objective measures too poor."

The actual testing would be carried out on 660 patients at the Derriford Hospital in Plymouth, and would use cannibis from the European Institute for Oncology and Immunological Research in Berlin.

The chemicals would be given in tablet form, not smoked.

In tandem with the test, patients would be able to compare the effect with that given by a chemical extract of cannabis called THC which is thought by many to be the key active ingredient.

If the the effects are the same, this might confirm that THC could be given alone - and this is already available, although it is not fully licensed in the UK.

Professor Moffatt said: "The drug is actually obtainable on prescription at the moment."

He said that he was still opposed to the medical excuse for full legalisation of the drug: "It certainly isn't a backdoor way at all. We are entirely focused on the medicinal effects."

The UK Government has already given permission for cannabis to be cultivated at a secret location for use in medical trials.