Cannabis has been shown to help ease the symptoms of some illnesses
It is part of an ongoing project by a UK pharmaceuticals company, based in Salisbury, Wiltshire, to test the effectiveness of cannabis in treating different conditions, including multiple sclerosis.
More than 100 people with terminal cancer will take part in the Phase Three trials at more than 20 UK centres, GW Pharmaceuticals said.
Cancer pain is a target market for GW Pharmaceuticals' programme of developing medicines derived from cannabis.
GW Pharmaceuticals executive chairman Dr Geoffrey Guy said: "This is a significant milestone for GW, and, we hope, for sufferers from cancer pain.
"Cannabis-based medicine has the potential to provide considerable advantages over current medications to cancer patients.
Promising results
"The potential market is very significant since approximately 40% of cancer sufferers at present have unmet needs in pain suppression."
The cannabis-based medicine will be administered by means of a sublingual - under-the-tongue - spray.
The Cancer Research Campaign (CRC) believes the trial is worthwhile.
The CRC's head of trials Kate Law said: "We've known for some time that cannabis type compounds help some people with intractable nausea and vomiting brought on by chemotherapy.
"This is the first that I've heard of it being tried in people with intractable cancer pain.
"The results, if positive, could make a big difference to many cancer patients."
GW Pharmaceuticals has also begun a new trial involving patients with brachial plexus injury, a severe form of nerve-damage pain which often results from motorcycle accidents.
Phase Three trials are the final stage in preparing for a medicinal product to be approved by the Medicines Control Agency.
Preliminary results from Phase Two MS and spinal cord injury trials have shown significant improvements in a range of symptoms.
David Blunkett said that if current clinical trials were successful the law would be changed to allow the use of cannabis-based prescription drugs.