Two MPs today backed calls to legalise cannabis for medicinal use, saying it would offer relief to thousands of sick and elderly people suffering from chronic pain.

The Labour MPs Paul Flynn and Brian Iddon were supporting a rally today in Parliament Square by the Cannabis Education Trust to raise awareness of the problems faced by medicinal cannabis users.

Mr Flynn, who has campaigned for the legalisation of the drug for medical purposes for 12 years, said he planned to reintroduce his private member's bill, first presented to parliament in 2001, to prevent the prosecution of chronically ill people.

"People around the world have testified in their thousands about the benefits of taking cannabis to relieve chronic pain," he said. "But because of our hang-up in this country with recreational use of the drug, we've condemned otherwise law-abiding citizens to risk jail."

He said there had been legal cases in which juries had let off people who said they were using cannabis medicinally.

"We must test the waters again. The law is an ass. Judges have called for parliament to revisit the issue."

Mr Flynn blamed the political parties' fear of being painted as weak on law and order for the failure to legalise cannabis for medicinal use.

"We had a bill last year that wasn't opposed by anybody to reclassify magic mushrooms as a class A drug - the same level as heroin, which is stupid because they're not at all that dangerous."

Many people with multiple sclerosis have used cannabis illegally to relieve their symptoms, including spasticity - muscle tightness and stiffness - and nerve pain. An estimated 85,000 people in Britain suffer from the disease.

Mr Iddon, chairman of the all-party parliamentary drugs misuse group, favours legalising cannabis for medicinal and recreational use, provided that in the latter case it is sold with clear health warnings.

The MP, a former chemistry lecturer, said it was "very wrong" that chronically sick patients had to choose between living in severe pain or risk themselves or one of their relatives being sent to prison for buying cannabis or growing it for medicinal use.

He said most medicinal cannabis campaigners were "normal people" and did not fit the "loony cannabis smoker" stereotype.

Earlier today, campaigners delivered a petition to Downing Street calling for an end to the prosecution of medicinal cannabis users.

Adam Slade, who suffers from chronic pain as a result of a congenital condition, said the current law on medicinal cannabis use put him in "an awkward position".

Mr Slade, who found standard painkillers ineffective at relieving his pain, said: "Cannabis improves my quality of life but because it's illegal it doesn't improve my quality of characterisation."

As well as decriminalising such use of the drug, the campaigners want a pain-relieving cannabis mouth spray to be made available on the NHS and cannabis clinics opened to provide patients with pain relief.

The mouthspray, Sativex, is already on sale in Canada to treat nerve pain but the company is facing a longer wait than expected for approval in Britain. Regulators in this country asked for additional data from the company last June.

Andrew Cornwall, coordinator of the Cannabis Education Trust, said the status of medicinal cannabis was a grey area of the law that needed clearing up.

"The needs of medicinal cannabis users are being neglected," Mr Cornwall, a lawyer, said. "There's no legal medicinal cannabis yet available in this country but many would argue that it was a medical necessity to provide the drug to chronically sick people to relieve their symptoms."