The Hong Kong government Tuesday began a controversial trial project to test 22,000 secondary school pupils for drug use.

Urine samples are being taken from pupils in 23 schools and results shared between teachers, police and social workers in a move to tackle a growing tide of youth drug abuse.

The scheme, described as voluntary, has triggered criticism with academics and youth welfare groups saying it unnecessary infringes civil liberties.

Pupils are selected for testing during normal school hours, heightening concerns that anyone who gives a positive test will be identified by the length of time they spend out of class.

The government brushed off the complaints, however, and insisted that it will be extended to all schools in Hong Kong if successful.

Under the plan, parental consent is required to obtain urine samples from pupils aged under 18, while the consent of both parents and students is required for pupils aged 18 or above.