The sites have experienced a steep rise in demand, with some regularly seeing more female users than male, the Telegraph reported.
Members are able to access the profiles of other would-be adulterers in their area and make contact through private messages. Though singles are also allowed to subscribe, the majority of users are married and middle aged.
MaritalAffair.co.uk, one of the largest sites of its kind, has almost 600,000 members.
The analysis also revealed most members are parents aged 35 to 54, university educated, and browse from their own homes. Women using the site on a weekly basis outnumber the site three to one.
Ashley Madison, a US-based website specialising in "discreet affairs" said it received a new British member every 45 seconds. More than 150,000 Britons use the site each week.
"The number of 50-plus married women in the UK joining us has surprised me. The foundation of our business in Britain is the married woman," Noel Biderman, the firm's chief executive, told The Times.
"I have been to London and a few other British cities where there are massage parlours and clubs with a wink towards gentlemen, but almost nothing for women," Biderman added.
The company said it acquired more than 10,000 new users the day after Valentine's Day, and most of them were married women.
There were similar increases after New Year's Day and Mother's Day last year.
A survey by the site found the capital of online adultery to be Manchester, where there is one subscriber for every 27 people.
Julian Brazier, MP for Canterbury and patron of the Conservative Christian Fellowship, said that extramarital dating sites were pushing Britain towards a "social breakdown".