Hong Kong Banking Employees Association vice-chairman Yu Siu-wa said there had been reports of the problem but added that staff who worked after 6.30pm were normally paid extra.

But Ip Wai-ming, deputy director of the Hong Kong Federation of Trade Unions rights and benefits committee, said it would be hard for employees to show evidence that they had been forced to work overtime without being paid.

Legislator Leung Yiu-chung said even if a contract stated that extra pay would be granted for working overtime, it was hard to prove that workers were being forced to stay behind after office hours. He said bosses could easily argue that staff chose to stay behind voluntarily.

Mr Leung said workers could refuse to do anything not stated in their contracts but faced the risk of being dismissed.

Nearly all banking staff surveyed supported a five-day working week but feared an extra day off might lead to longer working hours on top of their daily overtime if banks were not hiring more staff.

Of 602 banking employees interviewed by the Hong Kong Banking Employees Association and the Hong Kong Federation of Trade Unions, 98.2 per cent supported working five days a week, with weekends as regular days off.

More than half the interviewees feared that if they were made to work five days, they would have to work longer hours each day.

More than 80 per cent of respondents said that having Saturdays off could help mend family relationships and make it easier for them to arrange time to further their studies.

Mr Yu said working on Saturdays was not cost-efficient as the stock market was closed, leaving many documents to wait until Monday for further processing.

He said that since the government's announcement of a five-day week for civil servants, banks had been exploring the possibility for their industry, which has around 100,000 employees.

He also said the stress endured by banking staff had reached a critical level and on average staff had to work two hours extra each day.