The British government said on Saturday it had asked for an assessment of its decision to ease the rules on cannabis after studies showed the drug’s use may be linked to mental health problems.

Home Secretary Charles Clarke, a year after his predecessor controversially reclassified the drug, has written to the Advisory Council for the Misuse of Drugs, a statutory and non-executive public body, asking them to review the downgrade to a relatively low risk C category.

The move put cannabis on a level with tranquilisers and anabolic steroids.

That meant adults caught in possession of a small amount of cannabis could still be arrested, but the maximum penalty was cut to two years’ in jail from five.

Clarke said there was no indication that the use of cannabis had increased since the downgrade but added that studies had shown up dangers.

He mentioned a Maastricht University study that concluded that “cannabis use moderately increases the risk of psychotic symptoms in young people but has a much stronger effect in those with evidence of predisposition of psychosis”.

“I think there is merit in the Advisory Council assessing whether their position is at all changed by the emerging evidence,” Clarke wrote.

Opposition parties said the letter showed the government had recognised it had been wrong to downgrade cannabis.

“The downgrading of cannabis was a dreadful decision which sends out mixed messages about the dangers of drugs,” David Davies, the shadow home secretary, said in a statement.

Former Home Secretary David Blunkett defended the declassification last year, saying it would give police more time to tackle dealers and prosecute cases involving more serious drugs such as cocaine, ecstasy and heroin.