The research, conducted by the Trust for the Study of Adolescence, will alarm government ministers, who are concerned that the UK has the highest rate of teenage pregnancy in western Europe and that sexually transmitted infections (STIs), such as chlamydia, are found mainly in those aged 16-19.

The survey of 3,000 London secondary school pupils aged 15-18 found that:

· 39 per cent had sex for the first time when one or other partner was not equally willing

· Almost three in 10 lost their virginity for ‘negative reasons’, such as wanting to please a boyfriend

· 51 per cent of girls and 37 per cent of boys had had unprotected sex

· 58 per cent of girls and 39 per cent of boys had slept with someone at least once without using a condom

· Two in five wish they had waited longer before having sex

· Only 20 per cent who have sex for the first time take precautions, are in a steady relationship or feel the timing is right

Anna Martinez of the Sex Education Forum said. ‘These results show that there’s a lot of ignorance among young people about sex and that too many are becoming involved in high-risk sexual behaviour before they have had the chance to learn about issues such as how to resist pressure from partners, friends and the media to have sex. Schools, parents and professionals are failing young people by not giving them adequate support and information.’

The trust’s report, carried out for the Naz sexual health project in west London, reveals for the first time how sexual attitudes and experiences vary between ethnic groups. While 80 per cent of all the teenagers surveyed were ‘not sexually competent’ the first time they had sex, that figure rose to 93 per cent for boys of black-Caribbean origin, for example. And 32 per cent of boys of black African origin did not use contraception when they first had sex, compared to 10 per cent of white British pupils and 18 per cent of interviewees overall.

Many of those from ethnic minority backgrounds knew little about about how to prevent and identify the symptoms of STIs, and black Caribbean young men were more likely than others to have risky sex.

Bryan Teixeira, chief executive of the Naz project, said many young people from ethnic minority backgrounds ended up confused about sex because, while their parents often have traditional views, sex was discussed openly at school. Boys and young men were a particular problem, said Teixeira, as they were more likely to indulge in risky sexual behaviour and to have more partners than girls.

Although few places have as many pupils from an ethnic minority background as the Londoners in the survey – well over half – the results paint a worrying picture of the sex lives of Britain’s 5.2 million teenagers.

However, just 18 per cent of respondents had had sex before 16, the age of consent, which contradicts the impression of widespread underage sex and shows no increase on statistics from previous studies.

The report’s call for Sex and Relationships Education to be made compulsory in schools to help tackle what Teixeira calls ‘widespread knowledge gaps’ and to help pupils ensure their sexual welfare, was backed by CosmoGIRL magazine, which has been running a high-profile Just Say Know campaign.

Editor Celia Duncan said: ‘Some of the findings in this report are shocking and underline the case for all secondary school pupils to be taught not just about how to create a baby, but also about how to handle a guy who is pressurising you to have sex.

‘There are too many myths bandied about in the playground, such as ‘you can’t get pregnant your first time’. If pupils remain ignorant about sex, the consequences will be higher rates of STIs and unwanted pregnancies.’

A £50m Department of Health advertising campaign promoting responsible sex is due to start this autumn.

A teenager’s view

· Bethany Cole, a 16-year-old schoolgirl from Buckingham, thinks there is still not enough sex education.

‘Most people I know haven’t had sex yet and didn’t have sex before turning 16. They don’t care about breaking the law as they think no one will find out. Some male friends joke about sex and say that they’ve ‘had it’, but I suspect much of that is just male bravado. Me and my girlfriends do talk about sex; for example, if someone has had sex for the first time with a new boyfriend, or asks us if she can catch a sexually transmitted infection from something she did at the weekend. But my friends are pretty responsible, and use condoms, for example.

I’m lucky. I got a lot of sex education at school. Our form tutor gave one lesson over to answering questions that we’d all submitted anonymously, to minimise the embarrassment, which taught me a lot. Like how you can catch a STI or know if you’ve got one, and how to deal with peers or boyfriends who are pressuring you to have sex.

It’s crazy that many other schools, including one near mine, don’t give education on sex and relationships. It should be compulsory at all secondary schools – there’d be fewer teenage pregnancies, STIs and infertility caused by people leaving an STI untreated.

Some people say that if you teach young people about sex they’ll be more likely to do it. But I’ve been taught about the dangers of smoking since I was 12 and haven’t been tempted to try that. Proper sex education clues you up.’