The survey also found that internet pornography is the primary source of information about sex for half of the teens who responded. More than 30 per cent said they might have sexual fantasies or masturbate under the influence of online pornography.

Also, one-third said they were worried they might imitate online sex acts and commit crimes.

About 40 per cent of the youngsters did not think online pornography was misleading, and 65 per cent said they had downloaded porn photos or videos.

But in an alarming development, centre social worker Chiu Tak-choi said some youngsters had complained to him that their friends had put their photos in sexy poses on the internet without their consent, which had made them feel uncomfortable and embarrassed.

Mr Chiu called that a form of abuse: 'The teenagers who put the photos on the internet may think it is no big deal, but they don't realise they may hurt the people in the photos. They may have committed 'soft-core abuse' against their friends.

'It is worrying that some youngsters seem to be addicted to pornography and think of obscene scenes on different occasions.

'They may be misled by pornography and get the wrong information about sex, such as by thinking they should treat their sex partners brutally to excite them.'

One in five respondents also admitted they often downloaded pornographic photos or videos to their mobile phones or iPods, to watch at any time they liked.

Mr Chiu said the prevalence of browsing online pornography among young people was alarming.

He pointed out that 40 per cent of interviewees said they liked discussing sex with their friends on the internet, and 40 per cent also said they would like to imitate what they saw on online porn.

'However, in copying the actions in the videos, the teenagers may do something illegal,' Mr Chiu said. 'For example, they may commit crimes of indecent assault, such as becoming a peeping Tom or touching girls on the street.'

He urged teachers and parents to guide youngsters about using information from the internet properly. They should also provide formal sex education and counselling for teenagers, he suggested.

To help youngsters view sex correctly, the centre will launch a 'sex-peer counsellors' training scheme in September, which will provide workshops on sex education and attitudes towards dating to secondary school students as well as their parents.