A Bristol University professor has questioned the Government's move to reclassify cannabis from a Class C to a Class B drug.

David Nutt, the head of the Advisory Council on the Misuse of Drugs (ACMD), criticised ministers for effectively "criminalising people who are using a drug which doesn't harm other people, only themselves".

In 2004, the then Home Secretary David Blunkett approved the reclassification of cannabis from Class B, on advice from the ACMD.

Mr Blunkett said he hoped the move would free the police to concentrate on more serious drugs.

However, current Home Secretary Jacqui Smith said there was uncertainty over the drug's impact on young people's mental health.

The reclassification will mean higher jail sentences for the possession of cannabis.

David Nutt, professor of psychopharmacology at Bristol University, said: "Some individuals do get unpleasant mental reactions, but they are relatively small in number.

"We are not convinced that moving it to Class B with the possibility of five years' imprisonment for possession will have any beneficial effects.

"There are potential risks of criminalising people who are using a drug which doesn't harm other people, only themselves.

"Alcohol is more harmful [than cannabis] - to the population, certainly, and to the individual, possibly."

Cannabis has been illegal in Britain since 1928.

However, the law was applied in a rather erratic manner throughout the UK until the Misuse of Drugs Act in 1971, which introduced the drug classification system and sentencing guidelines.

In 2004, David Blunkett approved the reclassification of cannabis from Class B to Class C.

However, in May 2008 the current Home Secretary announced she would reverse that decision.

She said the move was necessary because of research linking heavy use of the drug with schizophrenia and other mental illnesses.

Ms Smith, who has admitted smoking cannabis at university, said she was particularly concerned over the rise in consumption of super-strength strains of cannabis, such as 'skunk'.

The Home Office say such strains account for 80 per cent of all cannabis seizures in the UK.

Home Office Minister Alan Campbell warned the average age of first-time cannabis users is now 13.

He said: "Cannabis is a harmful drug. I am extremely concerned about the use of stronger cannabis and the harm it can cause to mental health.

"We are reclassifying cannabis to protect the public and future generations."

In Class B, cannabis users who are successfully prosecuted can face an unlimited fine and up to five years in prison.

Marjorie Wallace, chief executive of the mental health charity SANE, supports the reclassification.

She said: "We have been campaigning for many years over the mounting evidence of direct links between cannabis and later schizophrenia.

"Cannabis, especially in its more toxic varieties such as skunk, can double the chance of developing severe mental illness in a significant minority of people, particularly the young whose brains are still developing."