Wang said she paid between 400,000 and 500,000 yuan (HK$465,000-$582,000) for the service.

“I think it was worth it, I’m satisfied,” she said.

So much so, she is now thinking of becoming a hunter herself.

“That way I can help women protect their families and their rights,” she explained.

The company Wang used, Weiqing – or “protector of feelings” – has 59 offices across the country, and offers free legal advice and lectures. Its founder Shu Xin said he has 300 agents at his command.

“My goal is to prevent divorces,” he said at his upmarket Beijing headquarters. “Every year we save some 5,000 couples.”

The mistress hunters are mostly women and are all psychology, sociology or law graduates. They spend three years learning the ropes before being sent out into the field, where they pose as neighbours, cleaners or even babysitters.

Ming Li, 47, has been doing the job for three years.

“I’m older than these mistresses, in general, so they listen to me,” she said.

“If the mistress goes to a park, to the supermarket or to work, I’ll happen to meet her. And even if she is a stay-at-home sort of person, I can claim I’ve got a leak in my apartment and ask for her help. We always find a way to initiate contact.

“One time, I pretended to be a fortune-teller, and the mistress asked me to tell hers. Obviously, I already knew all about her from the wife, so it was easy to leave her dumbfounded and exhort her to leave the husband. It was one of our most quickly resolved cases.”

Chinese divorce rates have surged from 1.59 per 1,000 people in 2007 to 2.67 in 2014, according to the most recently available civil affairs ministry figures – far higher than in Europe, with France at 1.9 and Italy at just 0.9.

In Beijing, official statistics show 73,000 couples divorced in 2015 – almost three times the number nine years previously.

“The reasons? The liberalisation of morals, tensions related to differences between the husband’s and the wife’s income, incompatible personalities,” said Zhu Ruilei, a divorce attorney at Beijing-based law firm Yingke. “But also the desire to pursue personal dreams is stronger than it used it be.”

According to a study by dating site Baihe.com, at least one party has been unfaithful in half of Chinese first marriages.

The survey found that more than 21 per cent of first-time husbands have had a mistress, and a similar number – 20 per cent – of wives have had a lover. In nearly 9 per cent of first marriages, both partners have cheated.

“Today being unfaithful has become easy, especially with the internet,” said Pan Xingshi, who runs an online advice company, referring to the popularity of Tantan, China’s equivalent of Tinder.

But mistresses are still poorly regarded in the country, where having children out of wedlock remains socially taboo. They are known as “xiaosan”, a derogatory term meaning a third person of lower rank than a wife.

Sometimes they fall victim to violent vigilantism.

In June, a video went viral showing a naked girl being attacked by a group of women. She was suspected of being the mistress of one of the women’s husbands.

“Mistresses are global. But specifically in China they are kept women: the husbands, often rich, pay for luxury apartments, cars and luxury products,” explained Weiqing chief Shu, a former journalist.

“Some women do not want to divorce out of fear of getting into financial difficulty. They just want to get rid of the mistress. That’s where we come in.”

It is an expensive process: the mistress hunters often have to rent similarly pricey accommodation and buy high-end jewellery and clothes as they try to forge a friendship with their targets.

“We are paid a lot. But we also risk losing a lot too, because if we fail then we repay the entire amount,” explained Shu, who says his mistress hunters sent 8,552 women packing in 2014 – some husbands have more than one.

Under Chinese law the activities of Weiqing and similar firms are not illegal, said Zhu, the lawyer, adding that they “serve a purpose”.

But there were also “many problems”, he added: “Invasion of privacy, the relationship between the mistress and the investigator is based on deceit. There is the risk that people’s feelings get hurt.”

Mistress hunter Ming has a solution for that: “Sometimes I help the mistress find a boyfriend,” she said. “It’s my way to bring her happiness.”