In defiance of medical advice and scientific opinion, the government is to press ahead with the reclassification of cannabis as a class B drug, possession of which will carry a maximum five-year prison sentence.

Home Secretary Jacqui Smith yesterday faced criticism that the she was dancing to a tabloid tune by ignoring the conclusion of the Advisory Council for the Misuse of Drugs that the health dangers from cannabis did not justify its inclusion in the higher category.

Announcing her decision, Ms Smith told the Commons that the dangers associated with skunk, the super-strong variety of the drug now in common use, vindicated her decision.

"I want it to be clearly understood that this powerful form of cannabis is an illegal and harmful drug," said Ms Smith. The scientific evidence from an ACMD report pointed to a "probable, but weak, causal link between psychotic illness, including schizophrenia, and cannabis use" but Ms Smith said the government was determined to lay down a tough line on misuse.

She added: "Where there is a clear and serious problem, but doubt about the harm that will be caused, we must err on the side of caution and protect the public."

Ms Smith said her decision took into account the "public perception" of cannabis, pressures on policing and the fact that 80% of cannabis seized on the streets is now the potent skunk variety. The drug was downgraded to class C during Tony Blair's premiership but Gordon Brown wanted to send a strong signal on taking office that the government does not approve of cannabis use.

Conservative Shadow Home Secretary David Davis welcomed the reclassification, but asked why it had all taken so long. "The government's lax approach to drugs is the hallmark of our broken society under Labour," he told MPs. "The UK has the worst level of overall drug abuse in Europe."

The most recent figures from the British Crime Survey show that cannabis use appears to have declined by 20% to 25% in the past five years in all age groups and that 2.6 million people aged 16 to 59 in England and Wales used cannabis in 2006/7.

The Scottish Drug Forum, which works to reduce drugs harm, said the changes were irrelevant because youth culture is a far more influential factor than legal penalties when it comes to young people making choices about cannabis. The organisation said more research into the true level of risk from cannabis will have more impact in preventing drug abuse.

Ms Smith said the system of police warnings for possession will be reviewed but this will have little effect on police procedures in Scotland where no system of graded warnings was introduced following the reclassification in 2004.

Detective Superintendent William MacColl, the police National Drugs Co-ordinator in Scotland, said the Scottish Crime and Drug Enforcement Agency has been consistent in charging for possession and targeting cannabis cultivation, regardless of reclassification.

In the Commons, LibDem Home Affairs spokesman Chris Huhne said, with irony, that the committee should be replaced with one composed of tabloid editors.