Up to 20,000 patients in the UK are to be given medical cannabis over a two-year period in an initiative that aims to create the largest body of evidence on the drug in Europe.

The move, to be unveiled on Thursday, is backed by one of the UK’s leading medical bodies and it is hoped it will persuade the NHS to prescribe the drug for a range of conditions.

Although medical cannabis was legalised in the UK a year ago, it remains unobtainable for many patients, according to campaigners.

“Medical cannabis is still out of reach for far too many,” said Professor David Nutt from the independent scientific body Drug Science, the organisation behind the launch of Project Twenty21, which will see 20,000 patients supplied with subsidised cannabis products by the end of 2021.

“Patients are left untreated, in significant debt from the cost of private prescriptions, or criminalised as they are forced to turn to the black market. They don’t deserve any of this, and the situation with prescribing desperately needs to change.”

The project – which will be launched at, and is backed by, the Royal College of Psychiatrists (RCP) – aims to create the largest body of evidence on medical cannabis in Europe, to convince policymakers that the drug should be made as widely available, and affordable, as other approved prescription medications.

Since legalisation, doctors have been wary about prescribing the drug to a lack of evidence about its efficacy. There are also fears it is being over-hyped by a nascent industry focused on maximising profit.

Project Twenty21 will study the drug’s effects on patients who have either chronic pain, epilepsy, multiple sclerosis, post-traumatic stress disorder, Tourette’s syndrome, anxiety disorder or who have had a history of substance misuse.

“The RCP hopes this project will address the paucity of evidence for the use of cannabis-based medicinal products in all health settings, including mental health,” said Professor Wendy Burn, the RCP president.

“We hope that this project, along with other research such as more much-needed randomised control trials, will continue to build the evidence on CBMPs”.

It is estimated that there as many as 28 million people in the UK will at some stage, suffer from chronic pain.

“Data from several countries reveal that medical cannabis have benefited several thousands of patients,” said Dr Arun Bhaskar, president of the British Pain Society. “There are more than eight million people with disabling chronic pain in the UK and medical cannabis is still out of reach for them. Trials like Project Twenty21 could provide evidence for safely and effectively prescribing these medicines that has the potential to provide pain relief and other life-changing benefits for some of these patients.”