A nyone who is tempted to rekindle old romances or start new ones on the internet be warned: Facebook is now being cited in almost 20 per cent of divorces.

A law firm in America has made the startling claim after discovering that nearly one in five of its clients named the social networking site in their petitions.

Divorce-Online's managing director Mark Keenan said he was "really surprised" to discover that out of 5,436 cases, 989 contained a reference to Facebook. "The most common reason seemed to be people having inappropriate sexual chats with people they were not supposed to," he said.

Around 14m people in Britain are thought to use social networking sites like Facebook. And the temptation to use them to get in touch with old flames or flirt with new acquaintances seems to be too great for many married people.

Suspicious partners are also finding more ways to check up on their errant spouses, and computer companies are developing programmes that enable them to monitor online activity.

More cases of online infidelity are coming to light. Last year Amy Taylor, a 28-year-old woman from Cornwall, split from her husband after discovering his alter ego was having an affair with an escort in the virtual world Second Life. Also last year a woman discovered that her marriage was over when her husband updated his Facebook status to read "Neil Brady has ended his marriage to Emma Brady".

Solicitor Karen Moores told the Daily Mail : "It started off with Friends Reunited where people were hooking up with exes from their school days, arranging to meet up and then starting affairs. Now it's Facebook, with people discovering their partners emailing or pictured with other people.

"If I was up to anything, I would never do it anywhere near the internet."