The Prime Minister last night appeared determined to press ahead with tightening up the law on cannabis despite reports that his official advisors are about to recommend against reclassification.

Gordon Brown's spokes man downplayed suggestions that the Advisory Committee on the Misuse of Drugs (ACMD) had concluded that there was no need to re-classify cannabis again - from class C to class B.

He pointed out how the reports appeared to be based on "one presentation" and that the ACMD had yet to reach its final conclusion. The spokes-man added that Mr Brown would take account of its recommendation once he had it.

At his regular Downing St press conference on Tuesday, the PM made his own personal view clear, saying: "If we are sending out a signal, particularly to teenagers and particularly to those at the most vulnerable age, young teenagers, that in any way we find cannabis acceptable, given all we now know about the change in the way that cannabis is being sold in this country, that is not the right thing to do."

Mr Brown asked the ACMD of 23 drug experts to review cannabis's legal status, set in 2004, in one of the first decisions he made after taking over as Prime Minister last year amid concerns over stronger forms of the drug.

The committee's expected recommendation was taken at a private meeting, which discussed new academic research about the links between cannabis and mental illness.

This supposedly produced no evidence that rising cannabis use in the 1970s, 1980s and early 1990s had led to increases in the incidence of schizophrenia. The ACMD's view will, however, cause concern among mental health charities who argue that skunk, a stronger form of cannabis, can trigger psychotic breakdowns.

Last night, the Conservatives claimed the UK Government's drugs policy was in chaos as they accused Mr Brown of again being incapable of making a decision and urged him to press ahead with reclassification.

David Davis, the Shadow Home Secretary, said: "This shambles has been caused by Gordon Brown's complete inability to make a decision. Rather than take action, he took the soft option of ordering a review. He has allowed the expectation that he will reclassify to gather momentum. Now he must act."

Annable Goldie, the Scottish Conservative leader, added: "We need to reclassify this drug today. As official data shows, the number of cannabis plants seized in Scotland has doubled in recent years. Labour has sent out entirely the wrong message."

The Association of Chief Police Officers also called for cannabis to be reclassified as a Class B drug.

On the counter side, how-ever, Rethink, the mental health charity, said Mr Brown should heed the committee's advice. "Gordon Brown should put aside his personal views on cannabis and accept the fact that it does not make sense to reclassify," said Paul Corry, its spokesman.

"Use of the drug has gone down since it was downgraded in 2004 and research by Rethink shows that only 3% of users would consider stopping on the grounds of illegality," he added.