Dramatic new evidence of the dangers of cannabis emerged today as government scientists warned that the most harmful "skunk" variety is flooding our streets.

The Forensic Science Service said that skunk now accounted for 75 per cent of cannabis seized.

It also revealed that seizures of all forms of cannabis have shot up by 42 per cent, with nearly 4,300 kilos confiscated by police and customs in the first half of this year.

The revelations will increase the pressure on the Government to restore cannabis to its former class B status, reversing its decision to downgrade it to a class C drug.

The powerful drug can cause mental illness, say medical experts, and has led to a number of killings by addicts. Prime Minister Gordon Brown has already announced a review of the drug's status.

The effect of skunk, typically two to three times more powerful than other forms of cannabis, was a prime concern which led to his decision and today's revelations will heighten fears about the drug's impact.

Dean Ames, head of the Forensic Science Service's drugs unit, said there was conclusive evidence that dealers were now concentrating heavily on the more powerful strain of cannabis.

"The market is now dominated by skunk. I would say that in the region of 75 per cent of the seizures that we now get are of skunk," he told the Standard.

"That is across all types of seizures, whether it be large amounts from suppliers, or small amounts possessed for personal use. Skunk is a more potent form of cannabis, but many users like it and that is the way the market is moving."

Figures sent to the Forensic Science Service by police and customs show that 4,281kg were seized in the first six months of this year. That compares with 6,032kg for the whole of last year, a 42 per cent increase so far this year.

Mr Ames said one reason for the surge in skunk was that cannabis was increasingly produced in Britain. Customs and police had made smuggling more difficult and growing numbers of foreigners are being brought to Britain by drug gangs to work in houses converted into cannabis factories.

He added: "There is now an easy way to grow cannabis in this country because of the availability of equipment, specialist lighting and a cheap labour source through foreign nationals.

"Most cannabis is grown here in soil, which requires the plants to be tended every day, and a lot of people are actually being brought in to do that."

Any change in the classification of cannabis, which was downgraded by former Home Secretary David Blunkett in 2004, will follow a review of its harmfulness by the Government's Advisory Committee on the Misuse of Drugs.

The committee's chairman, Professor Sir Michael Rawlins, said: "We will review all the evidence, but the extent of use of cannabis and, in particular, of skunk will be an important consideration."

The Government says it is concerned about the dangers of skunk, but insists that overall cannabis use has been declining despite the reclassification.

The decision to downgrade cannabis means that users face softer penalties - often no more than a police warning. It was intended partly to free police to concentrate on tackling more serious crime.

At the same time as reclassification, the Association of Chief Police Officers issued new guidance stating that those caught in possession should not normally be arrested. Instead, the guidelines say, the offender should be given a formal warning and have their drugs confiscated.

The main exceptions are when a person is caught smoking cannabis in public, when there is a problem with the drug in a local area, or if the offender is in possession of cannabis close to a school or play area. In these cases, the guidance says that an arrest should take place.

PUNGENT, POTENT AND DANGEROUS

* Skunk is a hybrid version of cannabis that is typically two to three times stronger, although some strains can be even more powerful.

* The drug's potency comes from the high levels of tetrahydrocannabinol (THC) that it contains. Its name derives from the pungent smell it produces while growing. It is produced from legally obtainable seeds.

* The level of THC in normal cannabis is usually between four per cent and six per cent. But skunk's concentration of THC is around 15 per cent and sometimes even as high as 20 per cent.

* A study published by The Lancet warned that cannabis users were up to 40 per cent more likely than non-users to develop a pyschotic illness. The study also warned that up to 800 cases of schizophrenia a year in Britain could be linked to the drug.

* The Association of Youth Offending Team Managers has raised concerns about the effect of cannabis, warning that use of the drug is "out of control" among juvenile criminals in some areas.

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