More than 15,000 people were polled about
their sex lives
Money worries and the distractions of social media mean people are having sex less frequently, researchers say.
A survey of more than 15,000 Britons found those aged 16-44 were having sex fewer than five times a month.
The figure compared with more than six times a month on the last two occasions when the official National Survey of Sexual Attitudes and Lifestyles was carried out, in 1990-91 and 1999-2001.
The study's authors say modern life may be having an impact on libidos.
Dr Cath Mercer, from University College London, said: "People are worried about their jobs, worried about money. They are not in the mood for sex.
"But we also think modern technologies are behind the trend too. People have tablets and smartphones and they are taking them into the bedroom, using Twitter and Facebook, answering emails."
She also said the survey suggested that couples aged 16 to 44 may be using online porn as a substitute for sex.
Men polled in 2010 to 2012 reported having sex 4.9 times a month and women 4.8. In both previous surveys the averages were over six for both sexes both times.
The latest survey, which is carried out every 10 years, shows slightly more teenagers are having sex before they reach 16 - 31% now compared to 29% in the last survey.
Among other findings:
The poll also asked older people about their sex lives.
It found 42% of women and 60% of men aged 65 to 74 had had sex in the past year.
Nonetheless, they were the age groups having the least sex - 2.3 times a month for men and 1.4 for women.
Among men, the 25 to 34 age group were having the most sex - 5.4 times a month - and for women it was the 16 to 24 age group, 5.8 times.
The average woman aged under 44 said they had had 7.7 partners over their lifetime, while for men it was 11.7.
The survey also asked participants about whether they had paid for sex in the past five years. Hardly any women had, but 3.6% of men admitted to it.
But the poll - the full details of which have been reported in the Lancet - also revealed the extent to which people are forced to have sex against their will.
One in 10 women and one in 70 men said they had experienced it.
However, fewer than half had told anyone about it and even fewer (13% of women and 8% of men) had reported the crime to the police.