Possession of cannabis for personal use could still lead to a criminal conviction despite reclassification.

The Association of Chief Police Officers (ACPO) has published details of how cannabis possession will be policed after reclassification.

David Blunkett will lay an order before parliament which will see cannabis classified as a Class C drug.

Under the guidelines released on Friday, possession of cannabis would "ordinarily not be an arrestable offence" although officers will be expected to use their initiative against users, arresting those who are either repeatedly found with the drug, or carry it near or in places where there are children.

"The proposed reclassification of cannabis will mean that officers will still have a power of arrest for simple possession," said ACPO drugs spokesman Andy Hayman.

The move has been opposed by campaigners who warn it will inevitably lead to calls for legalisation.

The National Drug Prevention Alliance is calling for "firm, fair laws against the use of illegal drugs" in order to protect young people.

"Research shows that whenever drug laws are relaxed the use of drugs increases dramatically," said a spokesman.

The home secretary welcomed the new policing guidelines despite claims that they send out a muddled message.

"The government is determined to support the police in tackling the problem of drug abuse with an effective and realistic approach," said David Blunkett.

"Cannabis will not be legalised or decriminalised. It is a harmful drug that is illegal and will remain illegal.

"The change in classification will enable the police to target Class A drugs - such as heroin and crack/cocaine - which cause the most harm to users, their families and communities, as well as enhancing our work to get people into treatment."

Ministers are defending the decision to reclassify the commonly used drug - pointing out that the move was recommended by the Advisory Council on the Misuse of Drugs.

Under the new guidance issued to police officers, most offences of cannabis possession by adults will result in a warning and confiscation.

"There will be a presumption against arrest, except where public order is at risk or where children are vulnerable," says the Home Office.

"The police will also ensure that those who repeatedly flout the law are arrested and dealt with."

But young people under 18 who are found in possession of cannabis will receive a formal reprimand or warning at a police station.

Those found in possession close to schools or youth clubs could face a criminal conviction.

Blunkett denied the government was going soft on drugs.

"The proposal to reclassify cannabis is just one part of our updated drugs strategy. Investment in tackling drugs is being increased by half a billion pounds over the next three years," he said.

"We are expanding treatment, providing effective education and support for young people through the FRANK campaign and targeting action on the most dangerous, problematic drug users."

But the move was attacked by the Conservatives as the "worst of both worlds".

"There is a case for legalisation, and there is a case for people getting off drugs. What there is not a case for is making them semi-legal," said shadow home secretary Oliver Letwin.

"These new guidelines are not clear, and will only cause further confusion for the police and for the public. This is not the fault of the police. It is the fault of the home secretary."

A spokesman for the organisation DrugScope warned that the policing recommendations were still only guidance and noted that the law was not yet changed.

She added that possession could still result in a five-year sentence and urged ministers to address the "dual message" that sends out.