'Many Hong Kong people who have set up businesses on the mainland have been hit hard by the economic downturn. Many have lost a fortune and are heavily in debt. On the other hand, through the many years of relationship, their mistresses have been able to save a small fortune,' he said.

'When these mistresses see that their 'husband' is getting old and failing in business ventures, some seek other opportunities, while others just change their hearts.'

Mr Cheung said his clients were even prepared to raise personal loans to finance investigations, even though they were already heavily in debt. Such investigations typically cost about HK$10,000 and take about a week to produce evidence.

Cheung Kwan-cheung, another private detective, said the economic downturn had reduced his business by about 15 per cent. But that did not mean people had given up their mistresses. In his experience, most investigations were still commissioned by women, but the caseload varied according to the season.

'We tend to receive fewer telephone calls [for help] during long holidays such as Easter and summer vacation,' Mr Cheung said. 'I suspect that women are tied up taking care of children during long holidays and cannot spare the time to check on their husbands.'

One man who is in his mid-40s and spends up to three days a week working in Shenzhen, where he has a girlfriend, said economic conditions had affected his lover's view of their relationship.

'She is definitely more interested in more commitment from me than before, not just money,' he said. 'I'm not going across the border as much, and I guess ... there is the chance our relationship could end.'

Cases of bigamy involving Hong Kong men and mainland women have risen in step with the mainland's modernisation since the 1980s as more men crossed the border for work or business. Many have taken mistresses - usually women from poorer regions.

Kwok Chi-ying, supervisor of the Caritas Family Crisis Support Centre in Choi Hung, Kowloon, said 30 per cent of clients seeking counselling wanted help with extramarital affairs. Seven of every 10 such cases involved someone on the mainland, and the trend had barely been affected by the economic climate.

She said that while 80 per cent of these clients were women, who saw themselves as victims, the rest were men, who often felt remorse for what they had done and wanted guidance on how to handle the situation.

'They still respect marriage, but many men do not see it as an issue to support mistresses if they can afford it, and many poor mainland women are willing to become a mistress for financial reasons,' she said.

Agnes Ng Kwok-tung, assistant director of the Christian Family Service Centre in Kwun Tong, said the situation was gradually changing.

'In the past, many mainland women had the impression that Hong Kong people were wealthy,' she said. 'Now, they have better access to news stories about Hong Kong, and many gain a better understanding of Hong Kong's situation through visiting [the city].

'They no longer make a decision so quickly to marry a Hong Kong man and have children with him.'

Sophia Kao Ching-chi, chairwoman of the Women's Commission, a government body set up to promote the interests and well-being of women in the city, said younger Hongkongers were the ones who moved up to the mainland for work or business, and many intended to have families with mainland women rather than keep them as mistresses.

Ms Ng said bigamy did not necessarily arise from personal indulgence, nor would there be a significant drop in cases during an economic downturn.

'In these extramarital affairs, the relationship is not always about money. Some of the parties do have genuine feelings towards each other,' she said.

David Cheung feels men should not always be blamed for affairs.

'In almost half of my cases, the wives should share much responsibility. They were either too nagging, or they were so busy with their own career or business, they did not pay enough attention,' he said.

'In many cases, the men are seeking a companion to share and talk about their own problems.'