The number of British women having lesbian encounters has quadrupled in the past 20 years, according to the largest-ever survey of Britain’s sexual habits.

The National Survey of Sexual Attitudes and Lifestyles, which was published on Tuesday, quizzed more than 15,000 Britons about their sex lives.

It found that nearly eight per cent of women had been with female partners, compared with just 1.8 per cent in 1990.

In contrast, the number of men having gay sex has remained stable over the same period at 3.6 per cent, according to the findings published in the Lancet medical journal.

Professor Kaye Wellings of the London School of Hygiene and Tropical Medicine, which co-ran the study, said there had been a “remarkable” change in women’s sexual behaviour since the first survey two decades ago.

“In some areas of sexual behaviour we have seen a narrowing of the gender gap, but in others we have seen women overtaking men in the diversity of their behaviour,” she said.

The trends need to be seen “against the backdrop of the profound changes in the position of women in society, the norms governing their lifestyles, and media representations of female sexuality”, she added.

British women have twice as many sexual partners over a lifetime as they did 20 years ago – the figure now is 7.7 on average.

Men also have more partners than before – about 12 over a lifetime, up from 8.6 – but the smaller increase suggests “a narrowing of the gender gap”, according to the study.

Elsewhere the survey found that Britons are having sex less often but are enjoying it well into their 70s.

The average Briton has intercourse less than five times a month, compared with more than six times a month a decade ago.

The study said the drop was explained partly by the fact that more Britons live alone than before, meaning they have “less opportunity to have sex”.

Many older Britons, meanwhile, said their age had not stopped them from having sex – some 60 per cent of men and 42 per cent of women aged between 65 and 74 said they had at least one sexual partner in the last year.

The new survey is the first in the project’s 20-year history to question people aged up to 74.

At the other end of the spectrum, about a third of both men and women in Britain are having sex before the legal age of 16, the survey found.

While attitudes towards gay sex have become more liberal, Britons have become more disapproving of adultery than they were 20 years ago.

About two thirds of Britons disapprove of non-exclusivity in marriage – up from about half before.

“We tend to think that these days we live in an increasingly sexually liberal society, but the truth is far more complex,” said Professor Anne Johnson of University College London, which also worked on the project.