Cannabis is the most common drug among young people

Professor Neil McKeganey warned against random drug testing until there is "clear data" on its effectiveness.

The government has given English schools the freedom to test and the Tories are pledging increased funding.

The report, published by the Joseph Rowntree Foundation, has called for more study into the impact of testing.

Positive test

Prof McKeganey, from the university's Centre for Drug Misuse Research, said: "Very few independent and rigorous evaluations have been conducted to identify the impact of drug-testing programmes in schools.

"Where studies have taken place, the evidence that programmes lead to a reduction in illegal drug use is far from conclusive."

His report said that imposing drug-testing programmes could undermine trust between pupils and staff, making it harder to discover whether a positive test is the sign of a significant drug problem, or experimentation.

It could also give rise to calls for staff to be tested as well, something teachers and their unions might resist.

He said tests were likely to identify pupils who used cannabis occasionally and could encourage some to switch to drugs which clear the body much more quickly, like heroin.

if random drug-testing programmes were to be piloted, there would be an obvious need to ensure that their impact was rigorously and independently evaluated Prof McKeganey

He also said testing programmes could lead to an escalation in attempts to conceal illegal drug taking rather than reducing its use.

The report notes that government guidance on drug-testing remains cautious about the development of random drug-testing programmes.

Schools contemplating their introduction are encouraged to gain agreement from the wider school community and consider whether the costs are an appropriate use of school resources.

'Negative outcomes'

Prof McKeganey said: 'It is difficult to judge the true likelihood of drug-testing being widely used in UK schools.

"Unlike the United States, no central government funding has been allocated for programmes.

"However, if random drug-testing programmes were to be piloted, there would be an obvious need to ensure that their impact was rigorously and independently evaluated.

'Such evaluation would need to be undertaken on a large enough sample of schools to be sure that any positive or negative outcomes were a genuine consequence of the drug-testing programme.

"Research would also need to consider the possible impact of a drug-testing programme on young people's wider educational experience."