A court in Japan has ruled that a husband who sleeps with another woman is not committing adultery if the sex is a business transaction.

The ruling was made after a man's wife sought ¥4m (£20,830) in compensation from a club hostess who had a long-term sexual relationship with her husband, the Japanese newspaper Asahi Shimbun reported.

Judge Masamitsu Shiseki, of the Yokohama district court, said the hostess had slept with the man only to ensure that he continued to be a member of the club she runs in the Ginza shopping and entertainment district.

The judge described the hostess's actions as being more comparable to prostitution than a love affair, and dismissed the wife's claim for emotional distress.

As long as the intercourse is for business, it "does not harm the marital relationship at all" the judge said. "Even if the wife is disgusted by the act, it does not constitute a legal offense," the judge added.

The wife did not appeal the ruling, and the case was finalised.

Prostitution in Japan In Japan having sex in exchange for money, or paying for sex, is illegal. But a loophole means that paying for any sexual activity which falls short of actual full sexual intercourse is not prostitution, so therefore is not illegal. Photojournalist Joan Sinclair, the author of Pink Box: Inside Japan's Sex Clubs, said this has led to the country's sex industry offering "absolutely everything imaginable but sex". The estimated annual turnover of Japan's sex industry is ¥2.3tr (£12bn).

The verdict was made in April 2014 but was only discussed in the current edition of legal magazine Hanrei Times, which covers court cases.

The seven-year relationship was described by the judge as makura eigyo which translates as "pillow business": a euphemism for sex work.

The practice refers to a hostess having a sexual relationship with a customer to ensure that he continues to visit the club regularly.

According to a 2008 poll conducted by Japan's Ministry of Health, Labour and Welfare, more than 20% of married men between the ages of 16-49 admitted to having had an affair recently, compared to 11% of women of the same age range.

Since a 1979 Supreme Court decision, anyone who knowingly has an affair with a married partner is obliged to pay compensation.

Japanese private detectives have made a lucrative business of proving adultery, but it is thought that this ruling would prove to be devastating to them, according to the Times newspaper.