Women are experiencing a second sexual liberation, every bit as controversial as that of the Swinging Sixties. Forty years after the pill set them free from the fear of unwanted pregnancy, a new generation are now changing sexual partners with a frequency that used to be thought the preserve only of men.

The second ever national survey of sexual behaviour in Britain, published in the Lancet medical journal today, shows that in the last 10 years both men and women have been choosing to sleep with more people than ever before, but the increase is more remarkable among women.

Among today's 16-24 year olds, 19.7% of men and 14.6% of women have already had 10 or more partners. Among women aged 25-34, the proportion with 10 or more partners rises to 22.7%. That is already higher than the figure among women 10 years older than them, the 35-44 year-old age group, at 19.4%.

Not only are younger people having more heterosexual partners, more people are experiencing gay relationships - one in 19 men and one in 20 women. And all kinds of behaviour that might lead to HIV and other sexual infections are on the rise. Researchers found one in seven men and one in 11 women were involved in more than one sexual relationship at one time. More men are paying for sex - one in 23 in the last five years, rising to one in 11 in London.

Infections

Anne Johnson, professor of infectious disease epidemiology at University College, London, and author of one of the three papers published today, said changes in social attitudes were part of the reason for the sex explosion. "We have become a more tolerant and less homophobic society. We have also become less censorious about one-night stands, but more intolerant of sex outside regular relationships. A higher proportion think of unfaithfulness in cohabitation as wrong," she said.

Television and films showing women in changing sexual partnerships had played a significant part in making it more respectable to have more than one lifetime relationship without being considered "a slag", she said. "We are a more open, honest and tolerant society in terms of sexual relationships than we were 10 years ago."

Sex is soaring - and so are sexually transmitted infections. The public health laboratory service released statistics yesterday which showed that cases of gonorrhoea, syphilis and chlamydia have all more than doubled in the last five years.

A second Lancet paper on the same study of 11,000 men and women aged 16-44 found that one in 10 adults had at one time caught a sexually transmitted infection. The most common is chlamydia. Most infected men and women do not notice any symptoms, but it can damage a woman's fallopian tubes and is now one of the commonest reasons of infertility.

Researchers carried out urine tests and discovered that one in 33 young adults of both sexes aged 18-24 is infected with chlamydia, and nearly all of them had no idea. Kevin Fenton, senior lecturer in epidemiology at University College London, said: "People with new partners, those who have had unsafe sex, or those with many sexual partners, should consider being screened for sexually transmitted infections."

Girls and boys are having their first sexual encounter earlier than ever before, a third research paper found. A quarter of women and nearly a third of men had sex for the first time when they were under 16. The average age of first sexual intercourse has dropped dramatically in half a century, from 21 in the early 1950s to 17 in the 1980s to 16 at the millennium.

Worryingly, many young women wished they had waited longer. Four out of five women and two out of five men aged 16-25 who had sex aged 13 and 14 regretted it. "Too many seem to have sex in circumstances that are not ideal," said Kaye Wellings, director of the centre for sexual health research at the London School of Hygiene and Tropical Medicine, one of the authors.

Although 80% said they used a condom the first time they had sex, the proportion was low among the youngest. Most young people said they get most of their information about sex from school - those who said they heard it from friends or even parents were more likely to have intercourse early.

Jan Barlow, chief executive of the Brook advisory clinics, said the good news about increased condom use and the success of school sex education masked a grimmer reality. Young people struggled to get access to free, confidential and accurate information about sex and contraception, she said, which was reflected in the fact the UK continues to have the highest teenage pregnancy rate in western Europe.

"There is a real danger that these survey results could lead to complacency around issues of young people's sexual health," she said. "Whilst the survey overall indicates a change in attitudes, towards a more tolerant view of issues of sex in the UK in the last 10 years, Brook believes that this change is not apparent in attitudes to young people and sex. There continues to be a climate of fear - almost taboo - around young people and sex, which acts as a barrier to sensible debate and to young people getting access to the information and services they need."

Nick Partridge, chief executive of the Terrence Higgins Trust, said better sexual health was within our grasp if higher quality sex education was provided in schools. In the face of rising HIV rates in the UK, he called for section 28 to be repealed, which forbids the promotion of homosexuality in schools, but which in practice stifles debate over safe sex and Aids. "Many parents are embarrassed to talk to children about sex, many teachers are restricted in what they can discuss and young people fall through the middle," he said.

Experimentation

Ann Furedi, of the British Pregnancy Advisory Service, said it was important we should understand why young people were having sex. "What happens is that very often we forget that young people have sex for the same reasons as older people, such as love or experimentation," she said. Policymakers and the health profession have not really decided whether it is young people having sex that we disapprove of or the adverse consequences of young people having sex, such as teenage pregnancies. It is very important to get that straight and tell young people - if you are having sex, have sex safely."