There are currently about 50,000 users of medicinal cannabis, receiving prescriptions from 33 private pharmacies.

However, very few people are being given the drug on the NHS.

The reason for this is that not all medicinal cannabis products are approved for use by the National Institute for Health and Care Excellence (NICE). Those which are, only use certain parts of the plant.

Specialist NHS doctors can prescribe unapproved products if they think the patient will benefit, but they have to ask NHS England to pay for individual cases and are often turned down.

Private specialists can prescribe products unapproved by the NHS, but this often comes at a high cost for patients.

A gram of legal medicinal cannabis in flower form costs between £5 and £10.

With an average patient consuming 0.5g a day, this can add up to as much as £150 per month.

The average price of illegal cannabis is widely reported to be about £10 per gram.

But legal clinics also require patients to attend regular check-ups, which can cost a minimum of £180 a year.

To qualify for a prescription through legal medicinal clinics, patients need to have first tried other medicines without success.

The Department of Health and Social Care said there was a need for more research on the effects of medical cannabis before any changes could be made to how it was prescribed on the NHS.

A spokesperson for NHS England said: “The NHS already offers several licensed cannabis-based treatments that have been proven to be clinically effective, approved by the medicines regulator and recommended by NICE.

“Many doctors and professional bodies rightly remain concerned about unlicensed products, and manufacturers should engage with the UK medicines regulatory process to seek a licence and provide doctors with the confidence to use their products.”