Cannabis should be legalised for medical use, a cross-party group of MPs and peers has recommended following a major inquiry.
The group's chairman Baroness Meacher claimed the refusal to recognise the drug's medicinal value is "irrational" while Green Party co-leader Caroline Lucas said it is "unacceptable" that people using cannabis to treat conditions risk prosecution.
Crossbench peer Lady Meacher said: "The findings of our inquiry and review of evidence from across the world are clear. Cannabis works as a medicine for a number of medical conditions. The evidence has been strong enough to persuade a growing number of countries and US states to legalise access to medical cannabis.
"Against this background, the UK scheduling of cannabis as a substance that has no medical value is irrational."
Lucas, who co-chairs the group, said: "Many hundreds of thousands of people in the UK are already taking cannabis for primarily medical reasons. It is totally unacceptable that they should face the added stress of having to break the law to access their medicine.
"This is a matter of compassion and human rights. The Government should have the political courage to view the issue of medical cannabis separately from any wider drugs reform and act urgently."
During the inquiry, the group took evidence from 623 patients, representatives of the medical professions and people with knowledge of how medical cannabis is regulated across the world.
The group commissioned neurologist Professor Mike Barnes to review evidence from the around the world.
His report concludes there is good evidence that medical cannabis helps alleviate the symptoms of chronic pain; spasticity - often associated with multiple sclerosis; nausea and vomiting, particularly in the context of chemotherapy; and in the management of anxiety.
Prof Barnes said: "We analysed over 20,000 scientific and medical reports. The results are clear. Cannabis has a medical benefit for a wide range of conditions.
"I believe that with greater research, it has the potential to help with an even greater number of conditions. But this research is being stifled by the Government's current classification of cannabis as having no medical benefit."
A Populus survey for the End Our Pain campaign, which is fighting for a change in the law, found public support for reform.
Some 68% supported allowing doctors to prescribe cannabis where they considered it would help their patients, with 11% opposed, 17% neither opposing nor supporting and 4% saying they did not know.
End Our Pain campaign director Peter Carroll said: "We estimate that over one million people in the UK take cannabis for medical reasons. All these people are at risk of police and court action.
"The poll shows that the British public understand that these people are patients, not criminals. We urge the Government to respond positively to the results of this poll, to today's parliamentary report, and the review of global evidence published alongside it."
Populus surveyed 2,068 adults online on June 22 and 23.