People who smoke high strength forms of cannabis on a daily basis are five times more likely to suffer psychotic illness than those who have never used the drug, according to a major new study.

Researchers said that eradicating the more potent types of cannabis from the market would be associated with a substantial cut in psychosis at a population level.

However, they also found that smoking any form of cannabis every day tripled a person's risk of developing psychotic symptoms, which can include disturbing visual or audio hallucinations and paranoid delusions that people are conspiring to harm them.

The findings, published at the Lancet, come after research found that cannabis use among 15-year-old boys in Scotland was the highest in the UK, and among the highest in Europe.

Meanwhile, rates of acute hospital admissions involving cannabinoids among all age groups in Scotland have doubled in just six years, from 10 to 20 per 100,000 population in just six years.

The new study is the first the analyse the links between the frequency and potency of cannabis use and incidence of psychosis at a population level. It covered 11 sites across Europe, including London, and one in Brazil.

The researchers estimated the local prevalence of psychosis by identifying all individuals who had presented with psychosis for the first time to mental health services between 2010 and 2015. These 901 patients were compared against a control group of 1,237 healthy participants, adjusting for factors such as age and socioeconomic status.

Information was gathered by the researchers on all participants’ history of using cannabis and other recreational drugs, and this was collated alongside published data on levels of THC - the main psychoactive compound - present in the cannabis circulating in each area. Cannabis was considered high-strength if it contained at least 10 per cent THC, which would cover the vast majority of street cannabis trading in London.

The study did not cover medicinal cannabis, where the active ingredient is a separate compound known as CBD.

Across the 11 sites, people who used cannabis on a daily basis were three times more likely to have a diagnosis of first-episode psychosis, compared with people who had never used cannabis. This increased to five times more likely for daily use of high potency cannabis.

In London - where high potency cannabis is widely available - the researchers estimate that it is linked to one in three new cases of psychosis.

If these high-strength forms of cannabis were no longer available, the incidence of psychosis in the UK capital would be expected to drop by 30% annually, they said.

There was no evidence of an association between psychosis and cannabis among people who smoked it less than once a week, however, regardless of potency.

Dr Marta Di Forti, lead author from the Institute of Psychiatry, Psychology, and Neuroscience at King’s College London, said: “Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms. They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level.”

The authors conceded that there are some weaknesses in the study - for example, it is based of self-reported cannabis use, not objective sampling from urine, blood or hair.

Dr Suzanne Gage, a psychologist at Liverpool University who has written comment article accompanying the Lancet, also stresses that it is not possible to say definitively that smoking cannabis caused psychosis. Previous research has found genetic correlations between individuals' risk of psychosis and their likelihood to use cannabis.

Dr Gage said: "It is possible that sub-clinical symptoms might have existed prior to cannabis initiation."

David Liddell, CEO of Scottish Drugs Forum, echoed that point.

He said: "We need to be careful here. I would urge caution – we have been here before with an alleged link between schizophrenia and cannabis use in the past.

"Research then showed that schizophrenia was declining in the general population when at the same time the use of high strength cannabis was increasing.

"If we accept the data, we have to be clear whether the relationship between high strength cannabis use and people experiencing psychosis is a causal relationship or merely associative. In other words, does this research data show that psychosis is caused by the use of this type of cannabis or that people who are, or have the potential to be psychotic are more likely to use strong cannabis but perhaps develop psychosis for some other reason altogether? This is a crucial distinction."

The Royal College of Psychiatrists in Scotland is currently undertaking its own review of cannabis use and mental health.

A spokesman said as a result they could not comment on the Lancet study, but hoped to publish their own recommendations in June.

Nick Hickmott, team leader at drug and alcohol charity Addaction said many people experimented with cannabis without any problems, but stressed that the problem was with potency.

Mr Hickmott said: People who regularly take lots of high strength cannabis are at risk of potentially serious harm. It can be particularly harmful for younger, developing brains. At the sharp end of the spectrum this can include psychosis, although in my experience this is rare.

“My advice is avoid using high strength cannabis every day and pay attention to how it makes you feel. If you end up feeling anxious or just unsettled then it might be best to give it a miss. It’s also not a good idea to mix it with alcohol or other drugs."