Police will not adopt a tougher approach to cases of simple possession of cannabis when ministers upgrade the legal status of the drug to class B, the Guardian can disclose.

The Association of Chief Police Officers (Acpo) confirmed last night that the current policy of "confiscate and warn" would continue, despite Gordon Brown's determination to reclassify the drug in an attempt to "send a tough message" to young people about its use.

Chief constables are debating whether or not fixed penalty fines should be available alongside cannabis warnings. But the basic approach of saving police time by not making an arrest and taking the offender to the police station to be charged, introduced four years ago, will remain.

Before cannabis was downgraded to class C in 2004, 58% of possession cases formally dealt with by police ended in arrest and formal caution, while 42% were taken to court.

Campaigners for drug law reform last night questioned the relevance of the drug classification system, which dates back to 1971, and its ability to send a message.

Roger Howard, chief executive of the UK Drug Policy Commission, and a former government drugs adviser, said: "There will be no new powers or resources for policing if cannabis is made class B, and cannabis warnings can still be issued instead of arrest."

He said this underlined the muddle at the heart of government over the purpose of a drug classification system which was unlikely ever to be able to "send a message to young people". Since cannabis had moved from class B to class C, the number of schoolchildren who think it is fine to try cannabis had halved, he said.

It is expected that Acpo guidance to police officers will use different language from existing guidelines to stress the discretion that is available to constables to take more robust action in cases involving repeat offenders or aggravating factors such as disorder or evidence of organised crime.

An Acpo spokesman said last night: "The key will be the discretion for officers to strike the right balance. We do not want to criminalise young people who are experimenting." However, he stressed that cases involving "aggravating factors" were more likely to see an arrest and prosecution.

When the police announced their support for regrading cannabis as a class B drug this year, Simon Byrne, Merseyside's assistant chief constable and the Acpo lead on policing cannabis, entered a little-noticed but crucial caveat to the police position. He said that since cannabis had been downgraded there had been growing concerns over increased potency, the rise of "homegrown" cannabis farms and a perception that its legal status meant it was seen as a low policing priority.

But he added that the police had supported the decision to downgrade the drug four years ago because of "the disproportionate time spent by frontline police officers in dealing with offenders in possession of small amounts of cannabis for personal use. Should the decision be taken to reclassify cannabis to a class B, Acpo believes the service should retain this flexibility in dealing with instances of possession on the street, including the discretion to issue warnings in appropriate circumstances".

The 2005 Serious and Organised Crime and Policing Act introduced new criteria for making an arrest which emphasised that it had to be necessary because, for example, the officer doubted whether he had been given a real name or a valid address by the offender. The number of cannabis warnings issued has spiralled to more than 100,000 since its legal status was downgraded; that forms an important part of the ability of the police to meet their national target for the number of offences brought to justice.

In legal terms, the move back to class B means the maximum prison sentence for possession will be increased from two to five years.