Experts say legalisation would reduce drug-related crime and mitigate the harmful effects of the drug on users, while raising up to £1bn annually in tax / Getty

The development is a major breakthrough in the treatment of Dravet syndrome - a rare form of the disorder

An experimental cannabis-based drug has successfully treated children with a rare, severe form of epilepsy. 

The successful clinical trial is the first of four final stage tests of the drug due to take place over the course of the year.

Drugmaker GW Pharmaceuticals is hoping to prove the therapeutic benefits of cannabinoids - the active ingredient found in marijuana. 

The company said in a press release on Monday that the 120-patient trial showed patients taking Epidiolex saw a median 39 per cent reduction of monthly convulsive seizures compared to 13 per cent taking the placebo. 

The trial was to treat Dravet syndrome, a rare and hard to treat form of epilepsy which usually begins in infancy and is characterised by behavioural problems, cognitive impairment, ataxia (unsteadiness) as well as prolonged seizures.

The company’s chief executive, Julian Gover, said: "This shows that cannabinoids can produce compelling and clinically important data and represent a highly promising new class of medications, hopefully in a range of conditions".

Mr Gover said they would now request a meeting with the US’s medicine watchdog, Food and Drug Administration (FDA), to seek regulatory approval to treat this form of epilepsy.

Mary Anne Meskis, Executive Director of the Dravet Syndrome Foundation, said: “Dravet syndrome is one of the most catastrophic types of epilepsy in children and safe and effective treatments are desperately needed. 

“We are thrilled to learn of these positive results, which bring much needed hope to the children and families who have been living with these debilitating seizures.”

GW was founded in 1998 to focus on using the therapeutic properties of cannabis to develop medicines. 

The London-based firm has a government licence to grow cannabis plants for its medicines in southern England.