Random drug testing should be brought into the classroom, Tony Blair has said.

The prime minister said new guidelines should be given to headteachers to deal with drug taking in schools and included should be provision for random testing of pupils.

"If heads believe they have a problem in their school then they should be able to do random drug testing," he told the News of the World.

"Guidance will be given to head teachers next month which is going to give them specifically the power to do random drug testing within their schools."

The Department of Education will detail the changes next month but the prime minister’s comments brought early support from teachers.

"I agree with the prime minister's statement on random drug testing but it will only work if the headteacher wishes to introduce it," David Hart, general secretary of the National Association of Head Teachers.

"It would give them another weapon in their fight against drugs being pushed or used in their schools."

"Whilst it would be quite wrong for it to be imposed externally, it could be a good example of school-police liaison that helps in the battle against the widespread drug culture amongst our youth."

But Conservative Tory education spokesman Tim Collins claimed the government was confusing its messages on drugs.

"This is a welcome announcement but it does seem inconsistent with the fact that the government has led children to believe that using cannabis is less serious than it was, due to its botched reclassification,'' he said.

"Once again, the prime minister is scrambling to repair the damage caused by his government's own flawed actions.''

Liberal Democrat education spokesman Phil Willis backed the idea.

"Drug abuse is a major social problem, endemic across society. It should not be treated in isolation as a school problem, nor should it be the responsibility of head teachers to test children," he said.

Campaigners questioned whether the action really would combat the problem of drug taking among young people.

Chief executive Martin Barnes of drug information charity DrugScope claimed the idea was excessive.

"We do not accept that drug testing and the random use of sniffer dogs are a proportionate or effective response to drug use among children and young people," he said.

"These measures risk driving drug use further underground, an increase in truancies and exclusions and a breakdown in trust between pupils and schools. An extensive process of consultation on the new guidance – with teachers, headmasters and experts in the field - showed strong opposition to drug testing and sniffer dogs. Despite this the government is now encouraging the practice."