A review of the decision to reclassify cannabis, after studies suggested links with mental illness, is not a policy reversal, the Home Office has said.
Cannabis was downgraded to class C in January last year
It was downgraded in January 2004 after advice from the Advisory Council on the Misuse of Drugs, which will be asked if new research would change its position.
The Tories said ministers were admitting they were "wrong". The Liberal Democrats welcomed the review.
The Home Office said the classification of drugs is under "constant review".
The move to downgrade the drug from Class B to Class C - placing it alongside steroids and some prescription anti-depressants - was made by former home secretary David Blunkett.
It made most cases of cannabis possession a non-arrestable offence.
Denying the suggestion that the review of the downgrading was an embarrassing reversal, a Home Office spokeswoman said: "It makes sense that government policy reflects scientific findings and is kept up to date."
She said the review would not affect the decision on whether to give the go-ahead to a prescription form of the drug.
Mental health charities welcomed the review, but charity DrugScope warned against the danger of the move being motivated by political, and not scientific, factors.
Home Secretary Charles Clarke, writing to the chairman of the advisory council, Professor Sir Michael Rawlins, 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.
In his letter, released by the Home Office, Mr Clarke also asked for advice about the issue of high-strength cannabis, known as "skunk".
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 "unquestionably harmful", but less so than other class B drugs.
Mr Blunkett said he welcomed the review, "which will avoid this important issue being misused and will enable a rational and sensible debate to continue".
The Tories have pledged to return cannabis to class B and shadow home secretary David Davis said Mr Clarke's move meant the government was admitting that downgrading the drug had been "the wrong decision".
Liberal Democrat home affairs spokesman Mark Oaten said his party had argued that the police should focus on "very serious drugs" and spend less time dealing with cannabis possession.
However, he added: "If evidence changes and it is clear that there are more harmful aspects of cannabis, it makes sense to review the law and that is why we would support any changes based on evidence that the home secretary is considering."