The number of schools that test their pupils for drugs is likely to rise this month as free urine test kits are offered to headteachers by a UK health company.
But ministers may be forced to ban drug-testing in schools later this year on the advice of a panel of European experts who say it is ineffective and a humiliating invasion of pupils' privacy.
Such a U-turn would be particularly embarrassing for Tony Blair, the Prime Minister, as he has called on schools to introduce the tests.
Although private schools have used drug-testing for several years, state schools began it only after Mr Blair gave it his personal backing in 2004 when, in an interview with the News of the World, he said: "If heads believe they have a problem in their school, then they should be able to do random drug-testing. They will have the power and the ability."
The Abbey school in Faversham, Kent, introduced random tests in January last year and within a month reported it had reduced drug-use.
But later in 2005 drug-testing by schools was investigated by the Pompidou Group, a wing of the Council of Europe which focuses on tackling drug abuse.
Its ethics committee met last autumn to draw up draft regulations for the practice. However, after hearing evidence from expert witnesses the panel decided that it should be discouraged altogether.
Angel Ruiz de Valbuena, a spokesman for the group, said: "The experts agreed on the unsuitability of any regulation on this matter, since in their opinion, drug testing shouldn't be allowed under any circumstance in European schools.
"Indeed, according to the members of this panel, the use of such tests in school may conflict with ethical principles such as respect for privacy, to the extent that they are unjustified intrusions by the state into young citizens' private lives that expose them to humiliating situations.
"Furthermore, the group considers that there is currently no scientific evidence of the effectiveness of drug testing in schools as a means of preventing drug use and abuse."
Ministers from 35 European countries, including the UK, will attend a meeting of the Pompidou group later this year where they will be asked to agree and sign a declaration, which is due to include a statement opposing drug testing in schools.
The statement would not outlaw the practice, but would cause embarrassment for countries where it takes place and could lend weight to legal challenges by pupils.
Drug-testing takes place in some Scandinavian schools and in a number of private schools in France, Portugal, Spain and Italy.
But members of the Pompidou group believe drug-testing in UK schools is particularly significant, even though only a very few state schools are doing it, because it is the only country where the practice has received public backing from the Prime Minister.
It is not known which minister will be the UK's delegate at the triannual ministerial meeting of the Pompidou Group, although it is likely to come under the remit of the Department of Health or the Home Office. Among the experts who presented evidence to the ethics committee was Artur Radosz of the European Coalition for Just and Effective Drug Policies, a network of more than 100 charities which work with drug-users.
Mr Radosz told the committee the tests were ineffective and expensive and that pupils could cheat them using information from the internet. "Drugs tests cost a lot, lead to invasion of privacy and can undermine relationships of trust between students and teachers and between parents and their children," he said.
"Young people who have the confidence of their parents and teachers, and are expected to assume responsibility for their actions, are the most likely in turn to act responsibly.
"Mass-media hysteria is sometimes leading decision-makers to seek easy answers."
Mr Radosz warned that pupils might switch from marijuana to more dangerous drugs they thought the tests would not detect such as inhalants and ecstasy. Innocent students who had bought legal drugs in pharmacies might also be punished because decongestants could produce a false positive result for amphetamine, while codeine could be mistaken for heroin.
A survey last year indicated that 82 per cent of parents and 66 per cent of children backed drug-testing in schools.
But pupils in the English Secondary Student Association have spoken out against them. Rajeeb Dey, the association's founder, said: "We want to improve the relationship between pupils and teachers. Drug-testing bodes badly for that."
The Abbey school began trialling drug-testing in January last year and selects up to 20 pupils each week at random by computer. They are given mouth swabs by specially-trained non-teaching staff. The samples are then sent to a laboratory to be checked for traces of drugs such as cannabis, cocaine and ecstasy.
So far 395 pupils have been chosen for tests. Only four refused and two tested positive, both of whom have been offered extra help rather than being expelled.
Peter Walker, the head, who retires next month, said the project had been a resounding success. He said: "The biggest reason for pupils to try drugs is peer pressure, but ours have got a persuasive excuse when offered them - 'Sorry, knowing my luck, I'll get tested on Monday'.
"I've stuck my head above the parapet on this one, but it's been one of many changes in the last year which have really helped to lift morale, and I don't think it is invasive."
The scheme has been copied by other schools including Colne community college in Essex and sparked international interest including from the Swedish parliament, which invited Mr Walker to address it in December.
Mr Walker said he had been encouraged by a report for the Joseph Rowntree Foundation by Professor Neil McKeganey, of Glasgow university.
However, this report warned that there might be unintended adverse consequences to drug-testing.
It expressed "concern that student drug testing has been widely developed within the USA, and may conceivably be so within the UK, on the basis of the slimmest available research evidence".