In October the UK will host what is believed to be its first medical cannabis conference, an international jamboree of weed-based therapies in London.

It may seem like an eccentric choice, given cannabis is illegal in Britain. But Saul Kaye, the Israeli entrepreneur behind the event, is confident the country is on the cusp of decriminalising marijuana and ushering in a “green rush” of investment. It is a phenomenon that has already gripped his homeland, creating a multibillion-dollar industry and around 500 companies. It is also one of the fastest-growing industries in the US, where medical marijuana is legal in some states.

Internationally the industry is worth around $20bn (£15.5bn) and is forecast to reach $100bn by 2020. The UK is a long way behind.

“If the UK doesn’t have a regulated cannabis industry in two years I’ll retire,” Kaye says confidently.

Cannabis therapies are not without controversy and there is conflicting evidence on their benefits, however. Anti-drugs campaigners also point to research showing that sustained use can lead to mental health problems and side effects including hypertension.

Supporters of the marijuana therapies say that they can help manage pain and also treat a wide variety of conditions, from epilepsy to Parkinson’s Disease. At a time when US drug companies face a flood of lawsuits from people claiming they have been harmed by opioid-based painkillers, medical marijuana is back on the agenda as a potentially safer alternative.