Despite the Advisory Council on the Misuse of Drugs (ACMD) concluding that the health dangers from cannabis did not justify it, Ms Smith told MPs it was right to “err on the side of caution and protect the public”.
A ACMD report said scientific evidence pointed to a “probable, but weak, causal link between psychotic illness, including schizophrenia, and cannabis use”.
In the population as a whole, the drug played only a “modest role” in the development of these conditions.
But, in a statement to the Commons, Ms Smith said a survey showed that 58 per cent of the public supported the upgrading of the drug from Class C.
Oldham East and Saddleworth MP Phil Woolas said: “It should never have been declassified. We have to take the views of the scientists and then make a judgement about what harm it can do.
“Ten years ago a study called altered Minds looked into this. I researched the issue with evidence from California which showed an alarming growth in head and mouth cancer among users and also mental health problems.
“While we would not want to criminalise young people for smoking a joint we need to be wary of its misuse.”
Ms Smith said the new classification will reflect the fact that much stronger “skunk” dominates the cannabis market, she said.
A study undertaken by 23 police forces — including Greater Manchester — provides evidence that skunk now makes up 80 per cent of seized cannabis, compared to 30 per cent in 2002.
Oldham West and Royton MP Michael Meacher added: “It is for the experts to decide. Politicians should not override what experts say unless there are overriding political reasons.”
The decision to reclassify cannabis is only the second time since 1971 that the Government has disregarded the council.
North-West MEP Chris Davies criticised the Government decision and said cannabis use had fallen since it was downgraded to a class C. He was fined £100 in 2002 for possessing cannabis in protest over the Misuse of Drugs Act after presenting himself at a police station asking to be arrested.
