Private investigators were hired during half the divorce proceedings in Britain last year as wives or husbands tried to establish whether the spouse was having an affair, a survey of leading law firms revealed today.

It found 49% of divorces in Britain last year came after one partner asked an investigator to check whether the other was committing adultery. This compared with only 18% in 2005.

The survey by Grant Thornton, a financial and business advice group, gathered information from 100 leading matrimonial lawyers. They said more people were citing a partner's extramarital affairs as the primary reason for the breakdown of the marriage, and fewer were complaining of emotional or physical abuse.

Last year 32% of divorces were attributed to infidelity, compared with 29% in 2005. In more than two-thirds of these it was the man who was declared unfaithful and in 31% of cases it was the woman.

The growing use of private investigators suggested that suspicion of adultery was even more prevalent than the practice of it. In nearly half of all divorces one partner used a private detective to look for evidence of infidelity by the other. Investigators were hired by 30% of divorcing women and 19% of divorcing men.

Andrea McLaren, head of Grant Thornton's matrimonial practice in London, said: "For the fourth year running our survey has shown that extramarital affairs are the primary reason cited for the breakdown of marriages in the UK. As this figure continues to rise it is little wonder that the number of individuals using private investigators increased."

The survey found only 4% of divorces were attributed to emotional and physical abuse last year, compared with 12% in 2005. Other causes last year were "behaviour" (17%), family strains (8%) and decisions of a personal nature, such as one partner wanting to have a child (4%).

Ms McLaren said she was surprised by a drop in the number of marriage partners trying to conceal assets during a divorce. Last year 10% of cases involved attempted concealment, compared with 16% in 2005. "Given the judgment in the Miller and McFarlane cases, which saw wives getting a larger share of the pot of wealth of their husbands, one would have expected this figure to increase," she said. A possible reason for the decline could be sophisticated forensic techniques to search for assets, making them harder to conceal.