One in four cases of psychosis in Britain is associated with a powerful "skunk-like" cannabis, a study has found.

Scientists found the potent form of the drug increased the likelihood that a user would develop the condition.

Casual users of the stronger class B substance have been found to be three times more likely to develop psychosis, while heavy users are five times more likely.

The findings of the six-year study - which have been published in the Lancet Psychiatry journal - suggest strong forms of cannabis can lead to serious mental illness.

Researchers from King's College London say there should be greater public awareness of the effects of using the stronger form of the drug.

Sir Robin Murray, professor of psychiatric research at the university, said: "It is now well known that use of cannabis increases the risk of psychosis.

"However, sceptics still claim that this is not an important cause of schizophrenia-like psychosis.

"This paper suggests that we could prevent almost one quarter of cases of psychosis if no one smoked high potency cannabis.

"This could save young patients a lot of suffering and the NHS a lot of money."

While skunk was associated with psychotic episodes, the milder form of cannabis known as hash had no such link, the scientists said.

The study looked at 800 people aged between 18 and 65 in south London, including 410 who had suffered psychosis and 370 healthy patients.

(Sky News)