One in 10 women in Britain say they have been forced into having sex against their will, the most comprehensive survey of Britons' sexual behaviour for decade reveals today, prompting a warning from researchers that sexual coercion may have become "normalised".

The findings from the third National Survey of Sexual Attitudes and Lifestyles (Natsal) survey, which questioned 15,000 people aged 16 to 74, also show that the proportion of women saying they have been victims of sexual coercion is more than double that of those who say they have been victims of rape.

Wendy Macdowall, of the London School of Hygiene and Tropical Medicine, the lead Natsal author, said that schools needed to address sexual coercion, which had become "normalised … with rape at the extreme end of the spectrum".

Those surveyed were asked "whether anyone has ever actually made them have sex against their will" and 9.8% of women said they had, at an average age of 18. For men the equivalent figure was 1.4%, according to the research, which is published in the Lancet today.

Macdowall said that there was a need for early intervention in schools to help address the problem "before those gender stereotypes are developing" and because "somebody who has been victimised at a young age is much more likely to be victimised later".

The Natsal proportion is significantly higher than that in the most recent Crime Survey for England and Wales, which was based on interviews with people aged 16 to 59 and found that 3.8% of women had been the victim of a rape since the age of 16. But the definition of rape is narrower, requiring the perpetrator to "not reasonably believe" that the victim consents.

Macdowall said the discrepancy was because people did not always realise that a crime had been committed: "We know that people who have experienced what would meet the legal definition of rape do not describe it as such. We've always known police reports are the tip of the iceberg and there's always been the suspicion the crime survey figures are low."

In 15% of cases among women and men recorded by Natsal, the perpetrator was a stranger, suggesting that coercion is at the heart of much abuse. Among female victims who were aged 13 to 15 when the event occurred, a family member or friend was responsible in nearly half of cases (45.2%), while for women aged 25 and over, a former or current partner was responsible in seven out of every 10 cases.

The responses also confirmed huge under-reporting by victims, with 12.9% of women saying that they had reported the matter to the police, compared with 8% of male victims. Natsal, which interviewed people between September 2010 and August 2012, also produced data on sexual behaviour, fertility, contraceptive use and sex-related diseases. The study follows previous ones in 1990 and 2000. But the most recent was the first to ask people about sex against their will.

The survey also found that one in six pregnancies are unplanned – another question asked by the survey for the first time.

The age of first sexual experience, at 16 (the median average), has remained the same as in the 2000 survey and the number of people having sex before the age of consent has not differed significantly either (31% of men and 29% of women). But with people cohabiting with a partner later and having children later, the researchers warned that there was a longer period when people were at more risk of "negative sexual outcomes" such as sexually transmitted diseases.

The biggest changes since the first survey are in behaviour reported by women. When the first survey took place, men had had more sexual partners than women and while that remains the case, the gap is narrowing. Men used to have their first heterosexual experience at a younger age but now it is the same for women.

Also, while the number of men reporting same-sex partners has changed little from 1990, for women it has increased fourfold, from 1.8% to 7.9% in the past 20 years. Researchers said this was not necessarily a result of female liberation, as championed by the likes of Madonna, but could be men demanding women act out common male fantasies of lesbian sex.

The report's authors say that sexual health services should be integrated and broadened. Professor Kaye Wellings, of the London School of Hygiene & Tropical Medicine, who co-led the study, said: "We need to start thinking about sex differently – sexual health is not merely the absence of disease, but the ability to have pleasurable and safe sexual experiences, free from coercion.

"Improving the quality of peoples' sexual experiences and their relationships will not just improve the effectiveness of sexual health programmes, but is also something that is important in its own right."