Nearly 40 percent of Hong Kongers having extra marital affairs are doing so in the mainland, Ming Pao Weekly reported.

The investigation, involving 1,400 residents of the Special Administrative Region, was conducted by the Family Crisis Support Centre operated by Caritas Hong Kong between 2013 and this year.

The survey showed that 40 percent of affairs occurred in mainland while 56 percent happened in Hong Kong .

Of those having illicit relationships, more than 60 percent are doing so with colleagues or friends while the rest are involved with sex workers or people met online.

"Chat lines such as WhatsApp and WeChat provide convenience for them," said Zhao Yulong, vice director of the Department of Social Work at the National University of Singapore and one of those in charge of the investigation.

The study also revealed that "lack of communication between wife and husband" and "more apart than being together" are two major reasons for people starting affairs.