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British people are having less sex now than in recent years, according to a large national survey.

The findings, published in the British Medical Journal, suggest nearly a third of men and women have not had sex in the past month.

That's up from around a quarter in 2001, according to the data from 34,000 people.

Less than half of men and women aged 16 to 44 have sex at least once a week, responses show.

Over-25s and couples who are married or living together account for the biggest falls in sexual activity across the 21-year period.

The data the researchers looked at came from three successive waves of the British National Survey of Sexual Attitudes and Lifestyles carried out in 1991, 2001, and 2012.

They give a snapshot of sexual behaviour among Britons.

According to the most recent survey:

Less than half (41%) of people aged 16 to 44 have had sex at least once a week in the last month

The proportion reporting no sex in the past month has increased - from 23% to 29.3% among women and from 26% to 29.2% among men between 2001 and 2012

The proportion reporting sex 10 or more times in the past month has fallen - from 20.6% to 13.2% among women and from 20.2% to 14.4% among men between 2001 and 2012

The average number of times that 35 to 44-year-olds reported having sex in the past month fell from four to two among women and from four to three among men

Why the drop?

Researchers from the London School of Hygiene and Tropical Medicine say the decrease in sexual frequency has been seen among people who have previously been sexually active, rather than more people deciding to keep their virginity.

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Although people under 25 and those currently single were less likely to be sexually active, the steepest declines in sexual frequency were among older married or cohabiting couples.

So are people simply going off sex? Apparently not.

Half of women and nearly two-thirds of men in the latest survey said they would like to have more sex.

This desire was more often voiced by people who were married or living together as a couple, which the researchers say "merits concern".

Too busy, tired or stressed?

Lead researcher Prof Kaye Wellings said the "sheer pace of modern life" may be a reason why many people are having less sex.

"It is interesting that those most affected are in their mid-life - the so-called 'sandwich' generation. These are men and women who are often juggling work, childcare and responsibilities to parents who are getting older."

Perhaps social pressure to over-report sexual activity may have eased, while gender equality means that women may now be less inclined to meet their partner's sexual needs irrespective of their own, say the researchers.

The decline coincides with increasing use of social media and a global recession, which may be other contributing factors.

Having less sex is not always a bad thing, says Prof Wellings. She said the survey results may be a comfort to many.

"What is important to wellbeing is not how often people have sex but whether it matters to them.

"Most people believe that others have more regular sex than they do themselves.

"Many people are likely to find it reassuring that they are not out of line."

Relate counsellor and sex therapist Peter Saddington said: "The important thing is quality not quantity. If you enjoy the experience you are more likely to do it again. But you have to make time for sex. It doesn't always have to be spontaneous. Putting a date in the diary can help."