Prohibiting cannabis for medicinal use is "absurd", Nick Clegg has said.
The drug was "much safer" than many medicines already used in the UK, the former deputy prime minister argued.
Writing an analysis piece published in The British Medical Journal (BMJ) the former leader of the Liberal Democrats added that there was "growing literature" supporting the drug's medicinal value.
The piece, co-authored by Baroness Molly Meacher, chair of the All Party Parliamentary Group on Drug Policy Reform, calls on ministers to reclassify the drug so patients can obtain medicinal cannabis.
A recent report for the All Party Parliamentary Group for Drug Policy Reform concluded that there was "good" evidence supporting medicinal use of cannabis for chronic pain seizures, nausea and anxiety.
And there was "moderate" evidence supporting its use for sleep disorders, appetite stimulation in the context of chemotherapy, fibromyalgia, post-traumatic stress disorder and some symptoms of Parkinson's disease, the report added.
"In this light, the UK's scheduling, denying any medicinal value for cannabis, looks ever more absurd," Mr Clegg and Baroness Meacher wrote.
They added: "Cannabis was placed in schedule 1 of the Misuse of Drugs Regulations, the schedule for dangerous drugs with no medicinal value, in 1985.
"This explicitly forbids doctors from prescribing it and inhibits research.
"The classification is irrational: firstly, cannabis has low toxicity and is much safer than many established medicines, not to mention two legal recreational drugs, alcohol and tobacco.
"Secondly, people have used the cannabis plant for its medicinal properties for centuries, if not millenniums. Recent years have seen the discovery of the human endocannabinoid system and a growing literature on the medicinal value of cannabis for specific conditions."
They said that the UK was in a "bizarre situation" where two components of cannabis - cannabidiol and tetrahydrocannabinol - were recognised for their medicinal value but the plant itself was still classed as "dangerous".
"Ministers urgently need to revisit the scheduling of cannabis and move the drug from schedule 1 to schedule 4 - which includes benzodiazepines, for example - in recognition of the limited risks and the medicinal value of the plant and its constituent parts," they added.
"This would facilitate research into the many conditions for which cannabis may be an inexpensive but effective treatment.
"But above all it would enable patients with a wide range of conditions to obtain cannabis medicines to alleviate their symptoms."
They also call for an end to criminal sanctions for the personal possession and use of all drugs.
Drawing on data from Portugal, where since 2001 the personal possession and use of all drugs have not been subject to criminal penalties, Mr Clegg and Baroness Meacher added: "British politicians should seriously consider introducing a version of the Portuguese model in the UK, involving a significant transfer of resources from criminal justice to treatment services."
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