A fifth of married people have a secret which could destroy their marriage, a survey has revealed.
While 25 per cent admit to having lied about something important, 42 per cent think they won't get caught out. But many still suffered sleepless nights, mood swings and constant guilt because of it.
Secrets kept by the 2,175 married Britons surveyed ranged from infidelity to money troubles and keeping pornography.
And the average marriage couple keep six secrets between them, according to the research by law firm Slater & Gordon.
It also revealed 26 per cent were certain it would be the end of their marriage if their secret came out.
While the average secret has been kept for nearly 10 years, a quarter of respondents said they had been hiding the truth about something from their other half for more than 25 years.
Amanda McAlister, head of family law at Slater & Gordon, said: "There is often a perception that marriages come to an end because of lots of little things coming to a head over time. But we find that a high proportion of clients come to us having been in a happy marriage just weeks earlier.
"It's normally the case that one lie or one secret is the undoing of the whole marriage."
Many were hiding secrets about money troubles, drug use, their sexuality or an incident where they had broken the law. But most said they had not told their partner because they did not want to worry them.
Other excuses were that their partner would not understand or the incident was a one-off that would not be repeated.
Nearly half of respondents said that someone else knew their secret and a quarter said they were scared it would one day slip out during conversation.