A fifth of married people have a secret that could destroy their marriage, according to research by a law firm.
While 25% admit to having lied about something important, 42% say they do not think they will get caught out - but many have still suffered with sleepless nights, mood swings and constant guilt because of it.
The secrets kept by the 2,175 married Britons included in the study ranged from infidelity to money troubles to keeping pornography, and the results show that the average married couple are keeping six secrets between them.
The research by Slater & Gordon also showed that 26% are 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 husband or wife 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.
"These divorce proceedings are often the most bitter and aggressive as well as there are a lot more emotions running high. They can also be quite acrimonious in terms of negotiating over assets and can drag on for years."
The most common secrets couples kept from one another were about infidelity, contact with a former partner, past relationships and pornography, the research found.
It also showed that many were hiding details of money troubles, drug use and an incident where they had broken the law, or their sexuality.
Most said they had not told their partner because they did not want to worry them, but 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.
Some respondents had suffered with panic attacks or turned to alcohol to get through their guilt, the study found.