The police and the Tories today demanded the reclassification of cannabis as a class B drug as the home secretary insisted she had yet to make a decision on the matter.

Assistant Chief Constable Simon Byrne, the Association of Chief Police Officers (Acpo) spokesman on cannabis, said that his organisation supported a return to a Class B classification "quite unequivocally".

Police chiefs say that their experience on the streets and the increase in cannabis farms makes the case for a return to a tougher law on cannabis, which was downgraded from a class B to class C drug in 2004.

"It is a very interesting debate and there are a wide variety of opinions which we respect, but the bottom line from a police point of view is that since reclassification four years ago we have seen a significant rise in cannabis farms, which point to increasing use of organised crime in this particular market; that's a worry for us," Byrne told BBC Radio 4's Today programme.

"There has been confusion on the streets about whether cannabis is legal or not which is fuelling public concern about how it is being policed."

The shadow home secretary, David Davis, said that the Tories would reclassify the drug, supply treatment for offenders and toughen up border police to prevent drugs from being brought in to the country.

"Not only is cannabis a gateway to harder drugs and a major cause of crime, it has real and tragic consequences for the mental health of so many people," he said.

But the home secretary, Jacqui Smith - who has admitted to past cannabis use when she was at university - was at pains to stress that a decision had yet to be taken.

In a letter, she assured the chair of the Advisory Council on the Misuse of Drugs (ACMD) that she had not already formed any conclusions about the classification of the drug.

The government advisory group was today starting a two-day evidence-gathering mission to investigate whether tougher penalties ought to be introduced for cannabis.

Along with examining the reclassification issue, the ACMD will hear new evidence on the latest trends in the potency of the cannabis available on Britain's streets, research into the drug's effects on mental health, and market research that looked at whether the public was aware of cannabis's current legal status and the potential penalties for using the drug

Some members of the ACMD said that they would consider resigning if the government ignored their conclusions.

Cannabis was reclassified in January 2004, making possession a largely non-arrestable offence, and placing it alongside some prescription anti-depressants and bodybuilding drugs.

Gordon Brown announced last July, within weeks of taking over as prime minister, that he would demand a new review of the drug's legal position because of emerging evidence about its effects and reports that stronger strains of the drug were becoming available.