Fewer than half of Britons have sex at least once a week and rates are declining, new data suggests.
People over the age of 25 and those who are married or living with a partner are having less sex than in the past, while the proportion wanting more sex is rising.
Researchers from the London School of Hygiene and Tropical Medicine analysed data for more than 34,000 men and women aged 16 to 44 who completed the National Survey of Sexual Attitudes and Lifestyles in 1991 (Natsal-1), 2001 (Natsal-2) and 2012 (Natsal-3).
The data showed a general decline in sexual activity in Britain between 2001 and 2012, with the steepest declines among the over-25s and those who were married or living together.
Overall, the proportion reporting no sex in the past month fell between the first and second surveys (from 28.5% to 23% in women and from 30.9% to 26% in men) but increased significantly in the final 2012 survey (to 29.3% in women and 29.2% in men).
The proportion reporting sex 10 times or more in the past month increased between the first two surveys (from 18.4% to 20.6% in women and from 19.9% to 20.2% in men), but fell in the final survey to 13.2% in woman and 14.4% in men.
Overall, 41% of men and women had sex once a week or more in the last month, the most recent survey showed.
Writing in the British Medical Journal (BMJ), the authors said: "Our data show that sexual frequency fell in Britain between Natsal-2 and Natsal-3.
"The most recent Natsal data show that fewer than half of men and women aged 16 to 44 have sex at least once a week.
"Those aged under 25 years and those currently single are less likely to be sexually active, but we saw the steepest declines in sexual frequency in those aged 25 and over and those married or cohabiting.
"At the same time, the proportion of men and women saying that they would prefer more frequent sex increased."
Men and women in better physical and mental health had sex more frequently, as did those who were fully employed and those with higher earnings.
The three surveys asked people about vaginal, anal or oral sex with opposite or same-sex partners.
The researchers noted that 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.
In this age group, the odds of reporting sex 10 or more times in the past month halved.
Meanwhile, in the first survey, 13.2% of women who were married or living together said they had no sex in the last month, dropping to 9.2% in the second survey but rising to 15.1% in the final survey.
Among men, the figures were 11.5%, 9.1% and 12.8%.
Among both men and women who were married or living together, the odds of reporting sex 10 or more times in the past month were roughly halved.
Declines of this magnitude were not seen among single people, suggesting the trend towards lower sexual frequency overall is largely due to the decline among sexually active married or cohabiting couples, the authors said.
However, the data also show that half of all women (50.6%) and almost two-thirds of men (64.3%) said they would prefer to have sex more often, particularly those who were married or living together, which the authors said "merits concern".
Lead author Professor Kaye Wellings said: "Several factors are likely to explain this decline, but one may be the sheer pace of modern life.
"It is interesting that those most affected are in mid-life, the group often referred to as the 'U-bend' or 'sandwich' generation.
"These are the cohorts of men and women who, having started their families at older ages than previous generations, are often juggling childcare, work and responsibilities to parents who are getting older.
"What is important to well-being is not how often people have sex but whether it matters to them.
"More than half of the men and women taking part in the study said they'd prefer to have sex more often, which could partly stem from unfavourable comparisons with what they think is the norm.
"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."
In the research paper, the authors also suggested that gender equality "may now extend to the sexual sphere".
They said: "Where women might previously have felt obliged to meet their partner's sexual needs irrespective of their own, they might now be less inclined to do so.
"Most compelling among the explanations, perhaps, given the age and marital status of the people most affected, relates to the stress and 'busyness' of modern life, such that work, family life, and leisure are constantly juggled.
"Life in the digital age is considerably more complex than in previous eras, the boundary between the private space of home and the public world outside is blurred, and the internet offers considerable scope for diversion."