Source: BBC News
A survey of 100 divorce lawyers found that 49% of break-ups last year involved a private detective compared with 18% the year before.
More women than men hired an investigator to prove or disprove their partner's infidelity, said business advisers Grant Thornton.
Adultery was cited in 32% of divorce cases last year, up from 29% in 2005.
Andrea McLaren, head of Grant Thornton London matrimonial practice, said they had expected some growth in the use of private detectives, but "had no idea that it would be such a significant increase".
"For the fourth year running our survey has shown that extra-marital affairs are the primary reason cited for the breakdown of marriages in the UK," she said.
"As this figure continues to rise, it is little wonder that the number of individuals using private investigators continues to rise."
Three women undergoing or considering a divorce hired a detective for every two men, the data suggests.
And where infidelity was blamed for a split, husbands were cited as the adulterers in 69% of cases.
The survey also found that the number of divorcing spouses who tried to conceal assets from their partner fell from 16% in 2005 to 10% last year.
Official figures released last August showed an 8% fall in the number of couples getting divorced.
There were 141,750 splits in 2005, the lowest for five years.