The Government's drug adviser launched a damning attack on Eton College yesterday for a hard-line drugs policy it described as "seriously misguided".
The rebuke to Britain's most famous public school comes in the wake of a decision to randomly drug test Prince Harry after the revelation he took cannabis last summer.
The Prince, who is third in line to the throne, faces expulsion from Eton if he is caught again.
Drugscope, a charity that advises on drugs issues and formulated the Government's drugs policy in state schools, said such a draconian policy could have a reverse effect and lead Prince Harry, 17, to take harder drugs.
Roger Howard, the chief executive of Drugscope, said: "Eton's policy of randomly testing their pupils for drug use and expelling any pupil testing positive is misguided.
"Quite apart from the civil liberties angle, there is no evidence that drug testing is an effective intervention in stopping drug use and it certainly does not distinguish between someone who tries cannabis once and someone with a real drug problem.
"Expelling pupils is likely to worsen any drug problem they may have and can also lead the child into a downward spiral of social exclusion."
According to Drugscope's figures, children excluded from schools are much more likely to end up with a serious drug problem. The research shows 63 per cent of excluded children have been offered cannabis compared with 25 per cent of children in school. Furthermore, 29 per cent of excluded children have tried a class A drug, such as heroin or cocaine, compared with only 5 per cent of children in school.
In its advice to state schools, endorsed by the Department for Education, it says drug testing leads to a "policy of confrontation" that "may seriously hinder the effectiveness of the school's drug education programme".
It says testing is "seldom reliable" because pupils can easily provide a false urine sample.
In prisons, where drug testing has been implemented for some years, anecdotal evidence suggests prisoners switch to hard drugs which stay in their bodies a shorter length of time to escape detection. Prisoners who test positive for drugs run the risk of having their sentences increased.
Dick Davison, the director of the Independent Schools Council information service, a body that represents public schools including Eton, said that random testing was only for pupils who had been identified as a having a drugs problem. It did not apply to all pupils.
He added that policies varied widely depending on the school and that there was no uniform policy across the private sector.
"There are still a number of schools who take a zero tolerance line with illegal drugs," said Mr Davison. In the last survey of schools, taken three years ago, 44 per cent of boarding schools automatically expelled pupils for illegal drug taking.Reuse content