Skunk, the powerful form of cannabis dominating the street drug market, is seven times more likely to cause psychosis than ordinary cannabis, scientists say.

Dr Marta Di Forti and Prof Robin Murray, who are among the authors of today's paper published in the British Journal of Psychiatry, say skunk is now the same price as ordinary cannabis on the streets of south London, where the study was carried out, and is now the one that is easiest for young people to obtain.

The researchers at the Institute of Psychiatry in London analysed the cannabis consumption of 280 people who were admitted with a first episode of psychosis to the South London and Maudsley NHS Foundation Trust, and compared them with 174 healthy people from the area. Those who used skunk were almost seven times more likely than ordinary cannabis users to develop psychotic illnesses such as schizophrenia, they found.

"In the last five to six years it has been established that there is a link between heavy cannabis consumption and psychosis, but there is a lot of argument about how important it is," said Murray.

Most cannabis users remain healthy, he said, but he pointed to the growing takeover in the street markets of skunk, which contain 18% THC, the substance thought to trigger psychosis, compared with about 4% in cannabis resin (hash).

Forti said 80% of the cannabis sold on the streets of south-east London was now skunk. People who use skunk do not necessarily appreciate its toxicity, she said.

Those in the study had an average age of 25. Those who had psychotic attacks heard voices and had paranoid delusions, such as a conviction that neighbours or family were conspiring against them.

About 1% of the population suffer psychosis at some time in their life. Studies have shown the risk of psychosis doubles in those who use cannabis a few times.