A plan to introduce HIV testing for children as young as 13 at schools in South Africa has been fiercely criticised by student and teacher unions.

The government believes that enabling sexually active pupils to know their HIV status could allow early access to life-saving treatment and help prevent the spread of the infection. But opponents of the voluntary programme say children may not be psychologically prepared to deal with a positive result or the stigma likely to follow.

The tests are expected to begin at secondary schools next month during weekends and holidays. Allen Thompson, deputy president of the National Teachers' Union, said: "We suspect we may be heading for disaster. Even parents are afraid to take HIV tests, so you can imagine a 13-year-old. Some will be afraid to say no to their teachers."

He accused the health department of using children to make up ground in its ambition for the world's biggest HIV testing scheme. Launched by President Jacob Zuma last April, the programme aims to have screened 15 million people by June but has fallen behind schedule.

South African officials have defended the scheme. Fidel Hadebe, a spokesman for the health department, said it had not yet been decided how the testing would be implemented.