People who smoke "skunk" — a potent form of cannabis — are almost seven times more likely to develop psychotic illnesses such as schizophrenia than those who smoke "hash" or cannabis resin, according to research.

Scientists from King's College London's Institute of Psychiatry said their study was the first to look specifically at skunk, rather than normal cannabis, and suggested high levels of tretrahydrocannabinol, or THC, were to blame for the drug's effect on mental health.

"The risk of psychosis is much greater among people who are frequent cannabis users, especially among those using skunk, rather than among occasional users of traditional hash," said Marta di Forti, the psychiatrist who led the study.

Di Forti and colleagues, whose work was published in the British Journal of Psychiatry on Tuesday, studied 280 patients who had experienced a first psychotic episode and 174 healthy people from the area of London where the research was conducted.

They found that those who had been diagnosed with psychosis serious enough to last a week and warrant admission to hospital were twice as likely to have used cannabis for longer than five years and more than six times more likely to use it every day.

And among all those who had used cannabis - from both the healthy group and the psychotic group - those with psychosis were almost seven times more likely to use skunk, a finding the researchers described as "striking".

The potential dangers of cannabis sparked a row between British politicians and scientists last month after the government sacked its chief drugs adviser for arguing that cannabis was no more harmful than alcohol.

Previous studies have suggested smoking cannabis can double the risk of developing psychosis, Di Forti said, but hers is the first to look at skunk - a drug she said was now taking over from cannabis resin in the illegal drug trade in many countries.

The two main constituents of cannabis are THC - the psychoactive ingredient which can produce psychotic symptoms such as hallucinations, delusions and paranoia - and cannabidiol.

The researchers said cannabidiol appeared to have anti-psychotics properties and could be counteracting the THC.

Skunk traded illegally in southeast London, where the study was carried out, has around 12 to 18 percent THC and 1.5 percent cannabidiol, while regular cannabis resin has an average THC of around 3.4 percent and an equal amount of cannabidiol.

"It seems that with hash the equal amounts of THC and cannabidiol may be reducing the effect, but with skunk that balance is not there," Di Forti told Reuters after a briefing.

"Unfortunately skunk is displacing traditional cannabis preparations in many countries," she said, adding that while skunk had been more expensive than hash in the past, it was now selling for a similar price - under 5 pounds ($8.24) a gram.