The Chief Executive of the Standing Conference on Drug Abuse, Roger Howard, said there was a "potentially huge" problem with poor quality testing which could lead to false results.

He urged schools and employers not to adopt "knee jerk" policies such as expulsions and sackings when tests proved positive.

The School Standards Minister, Estelle Morris, provoked controversy when she advocated a similar approach in a speech last year.

Ecstasy is said to be one of the drugs most commonly used by schoolchildren

A recent survey by the Headmasters' and Headmistresses' Conference suggested that 72% of boarding schools and 28% of private day schools used some form of drug testing. The proportion of state schools carrying out such tests is unknown.

Mr Howard told a conference on drugs in London that testing should only be carried out when it was "absolutely justified" and when the organisation had arranged ways to offer treatment to people recognised as having a drug problem.

"Drug testing technology is developing faster, perhaps, than our policies for responding to positive results," he said.

"We therefore risk seeing knee-jerk reactions where people are excluded from the workplace, from schools or from their families after testing positive."

Treatment regimes must be put in place before a testing programme begins, added Mr Howard.

The government's "Drugs Czar", Keith Hellawell, also addressed the conference.

He said a procedure for drug testing should be "an integral part of a business's health and safety policy", but warned that it must have the agreement of employees.