Prime Minister Gordon Brown was set for a showdown over cannabis after his drugs advisory board was reported on Thursday to have voted against his desire to tighten the law against it.

The BBC said the Advisory Council on the Misuse of Drugs (ACMD) had concluded that cannabis should remain on the lowest "C" classification of restricted drugs.

Brown announced in July that the government was considering whether to return the drug to a class B substance, after concerns about the potential mental health effects of stronger kinds of cannabis such as "skunk".

The government asked the council to assess the medical and scientific basis of the drug's C ranking.

At his monthly press conference on Tuesday, Brown said there was a stronger case now for sending out a signal that cannabis was "not only illegal but it is unacceptable".

Home Secretary Jacqui Smith - who has admitted she smoked cannabis while a student at Oxford - told BBC radio she could not comment as she had not seen the council's findings.

"I think the Prime Minister was right to ask the advisory committee whether or not we have got the classification right", she said.

"I do believe that cannabis use is serious which is why it's illegal."

Cannabis was lowered to class C in 2004, reducing the maximum penalty for possession to two years from five, to give police more time to deal with harder drugs such as crack cocaine.

On its present classification, police are unlikely to arrest adults caught in possession of cannabis, except in cases such as repeat offending, smoking the drug in public or having the drug near premises used by children.

Young people found with the drug will be arrested but are likely to receive a reprimand or warning rather than be charged.

The Home Office said arrangements for publishing the advisory council's report and the government's response had yet to be decided.

"The ACMD is due to give its recommendation to the Home Office this later this month and the Home Office decision on cannabis classification will follow shortly after," it said.