Research has found that almost 3 per cent of the adult population uses cannabis to treat a medical conditionLars Hagberg/AFP via Getty Images

Nearly a quarter of a million people with arthritis, 100,000 cancer patients and 50,000 multiple sclerosis sufferers are among 1.4 million unwell Britons who are being forced to buy cannabis illegally to treat their symptoms.

A landmark YouGov poll of more than 10,000 people has found that almost 3 per cent of the adult population uses cannabis to treat a medical condition, with usage across all age groups, social classes and genders.

More than half are using the drug every day and the average spend is £163 a month. This means patients are spending more than £2.6 billion a year on black market cannabis.

Medical cannabis has been legal in Britain for more than a year but strict conditions mean that only a handful of…