The Home Secretary has ordered a review of the decision to downgrade cannabis, as new studies suggest a strong link between the drug and mental illness.
Cannabis was downgraded to class C in January last year
Cannabis was downgraded from class B to class C in January last year, based on a recommendation from the Advisory Council on the Misuse of Drugs.
Charles Clarke has now asked the body to consider whether the fresh research would lead it to change its position.
The Tories said the government had recognised "they got this wrong".
Mental health charities welcomed the review, but charity DrugScope warned against the danger of the move being motivated by political and not scientific factors.
Mr Clarke, writing to the chairman of the advisory council, Professor Sir Michael Rawlins, said since the downgrading of cannabis - which made most cases of cannabis possession a non-arrestable offence - there had been no proven increase in use of the drug.
However, he highlighted concerns over several studies that strongly linked cannabis use to the development of psychosis.
A study by New Zealand scientists, published earlier this month, suggested smoking cannabis virtually doubled the risk of developing mental illnesses such as schizophrenia.
Mr Clarke wrote: "I want to be clear what influence the evidence presented within these studies has on the overall assessment of the classification of cannabis.
"I think there is merit in the advisory council assessing whether their position is at all changed by the emerging evidence."
In his letter, released by the Home Office, Mr Clarke also asked for advice about the issue of high-strength cannabis, known as "skunk".
He said the Dutch government was looking into whether cannabis above a certain strength should be given a higher classification.
A Home Office spokeswoman said the possibility of a two-tier classification system for cannabis in the UK would be a matter for the review, but added: "At this stage there is nothing to inform that."
The advisory council, which is made up of scientists and medical experts, said in 2001 that cannabis was "unquestionable harmful", but less so than other class B drugs.
"We said at the time of reclassification that cannabis would always be kept under review," the Home Office spokeswoman said.
"It is important to keep absolutely up to date on the position and take advice regularly from the scientific and medical experts. Our position will always be informed by that."
Putting cannabis in class C places it alongside steroids.
Former Home Secretary David Blunkett, who took the decision to downgrade cannabis, said: "I welcome the review which will avoid this important issue being misused and will enable a rational and sensible debate to continue, informed by the best scientific evidence and advice."
The Tories have pledged to return cannabis to class B and the party said Mr Clarke's move meant the government was admitting that downgrading the drug was "wrong".
Shadow home secretary David Davis told BBC Radio 4's Today programme he welcomed the move, adding: "The wrong decision was taken before.
"It is clearly part of a seriously flawed drugs policy that is causing all kinds of problems for the country.
"A lot of young people have had their lives destroyed by this drug, by the psychiatric damage it does and because some of them go on to harder drugs."
Former drugs tsar Keith Hellawell, who resigned over the re-classification issue, told Today: "Charles Clarke is trying to put right something that was put wrong 15 months ago."