Mr Brown said some stronger forms of cannabis could be lethal
Prime Minister Gordon Brown has said he wants to "send a message" to youngsters that using cannabis is "unacceptable".
Mr Brown, who is examining a report on potentially tightening the law, said he was worried about the "more lethal" use of skunk - a strong form of the drug.
The Advisory Council on the Misuse of Drugs, is expected to say cannabis should remain a Class C drug.
But Mr Brown told GMTV: "We really have got to send out a message to young people - this is not acceptable."
Cannabis was downgraded to class C drug - which includes substances such as anabolic steroids - from class B, in 2004. It means possession of the drug is treated largely as a non-arrestable offence.
But when Mr Brown became prime minister he announced a review by the advisory council, amid concerns that more harmful forms of the drug had become available.
A final report by the group, which plays a key role in setting Britain's drugs policy, was delivered to the Home Office on Monday and it is expected to recommend cannabis remain in the lowest category.
But Downing Street has already indicated that Mr Brown remains determined to tighten the law, against the advice.
The prime minister told GMTV: "I have always been worried about cannabis, with this new skunk, this more lethal part of cannabis.
"I don't think that the previous studies took into account that so much of the cannabis on the streets is now of a lethal quality and we really have got to send out a message to young people - this is not acceptable."
He added that he had only just received the report and would publish the results "very soon".
But he added: "I think people know my view about cannabis and particularly about this lethal version of it, skunk."
For the Conservatives, shadow home secretary David Davis said Mr Brown had "dithered" over what to do about cannabis, which could cause "real harm" to a young person.
"The demotivating and debilitating effect cannabis can have at the most crucial stage in a young person's development can seriously harm their life chances," he said.
Liberal Democrat home affairs spokesman Chris Huhne said the advisory council's recommendations should be published, accusing the government of "playing politics" with the issue.
He added: "There should be nothing secret about the scientific advice being given to ministers on this important issue."
Home Office research published in February suggested skunk, the strongest type of cannabis, now dominates the UK market - comprising 70% to 80% of samples seized by police.
Marjorie Wallace, of the mental health charity Sane, welcomed Mr Brown's comments saying the "strongest signals" about the dangers of cannabis must be sent to those at risk.
She said: "The front-line experience of organisations such as our own and others is that use of the drug can cause harm, not only to young people but to their families, making the outcomes worse for those with mental illness and robbing young people of their motivation and future."