Downing Street has signalled Gordon Brown remains determined to tighten the law on cannabis - against the advice of an independent panel of experts.
BBC News has been told the Advisory Council on the Misuse of Drugs believes that cannabis should remain in the lowest category, Class C.
Conservative Leader David Cameron called on Gordon Brown to tighten up the law on cannabis.
Mr Cameron said the prime minister must "stop dithering".
A Downing Street spokesman said reports that the Advisory Council were going to recommend keeping cannabis in Class C were premature.
He said Gordon Brown stood by earlier remarks about the need to signal that cannabis use was illegal and unacceptable.
The council - the official body which advises the government on drugs policy - was asked by the government to review cannabis's legal status amid concerns over stronger forms of it.
Its decision leaves the government in an awkward position, said BBC home affairs correspondent Danny Shaw.
If the government went ahead and reclassified cannabis to class B, ministers would be rejecting the findings of the council's panel of 23 drug experts - which has never happened before on a decision about drug classification.
The council's chairman Professor Sir Michael Rawlins refused to confirm or deny a decision, and said a report would be sent to Home Secretary Jacqui Smith later this month.
Home Secretary Jacqui Smith said she would not comment on the review until she had received it.
The prime minister's spokesman said: "As we understand it, this report is based on one presentation that was given to a meeting. The advisory council themselves are still some way away from reaching final conclusions.
"As to what the prime minister thinks, he made that fairly clear in the press conference earlier this week."
The Conservatives want the government to tighten up the law, regardless of the recommendations from advisers.
"There are all sorts of cannabis on the streets today. Skunk and super skunk are incredibly powerful and can lead to people having all sorts of mental health problems", the Conservative Party leader David Cameron said.
"The Conservative Party has a very clear view that it should be class B."
But the Liberal Democrats are urging the opposite. Their home affairs spokesman, Chris Huhne said ministers should "stop playing politics" and listen to the advice of the medical experts.
"The Advisory Council is the most authoritative and independent body capable of making a judgment, and we should not turn drugs classification into a political football or the plaything of tabloid newspaper editors", Mr Huhne said.
A meeting of the Advisory Council is reported to have discussed significant new research from Keele University about links between cannabis and mental illness.
The study found nothing to support a theory that rising cannabis use in the 1970s, 1980s and early 1990s led to increases in the incidence of schizophrenia later on.
Cannabis was downgraded from a class B drug to class C in January 2004.
MAXIMUM DRUG PENALTIES
Class A: Seven years for possession, life for supplying
Class B: Five years for possession, 14 for supplying
Class C: Two years for possession, 14 for supplying
The move to downgrade cannabis to Class C in 2004 was intended to free up police time and allow officers to concentrate on tackling harder drugs.
Adults found carrying cannabis are unlikely to be arrested, and young people are most likely to be arrested and reprimanded.
Possession of the class C drug does carry a maximum penalty of a two-year prison sentence, but this is rarely served. People found to be carrying a class B drug can be given up to a five-year sentence.
Former government drugs advisor Keith Hellawell said the government should never have reclassified cannabis to Class C.
He said the move created confusion for police, teachers, parents and young people by sending out the wrong message.
Police chiefs want cannabis to return to Class B.
The Association of Chief Police Officers (Acpo) said it stood by its recommendation made to the Advisory Council that cannabis should be restored to the category of a Class B drug.
But Steve Rolles, of the Transform Drugs Policy Foundation, said increasing jail sentences from two years to five through reclassification was not the best way to send a strong signal to teenagers about the dangers of the drug.
"Rather than mass criminalisation of millions of young people, the best way would be to invest in effective, targeted public health education," he told BBC Radio 4's Today programme.
HAVE YOUR SAY
Let adults make their own minds up, I choose to smoke cannabis and will continue to do so, regardless of classification
Gadge, North Tyneside
Mental health charity Sane was one group which gave evidence to the advisory group.
Marjorie Wallace, the charity's chief executive, said not enough was yet known about the direct links between cannabis and the brain.
She said she knew of hundreds of cases where people smoked cannabis heavily, in particular skunk, and went on to suffer psychotic breakdowns, hallucinations and paranoia.