Tens of thousands of foreign maids in Hong Kong are in "forced labour", according to a new report that fuels growing criticism of the city's treatment of its army of domestic workers.

The study by the Justice Centre estimates that one in six, or 50,000 of Hong Kong's more than 300,000 migrant domestic workers -- mainly from Indonesia and the Philippines -- fell into the "forced labour" category.

Its findings come after a report by the UN Committee Against Torture in December urged Hong Kong authorities to reform laws in order to protect victims of forced labour and trafficking.

The plight of the city's domestic workers was also thrown into the international spotlight by the high-profile abuse case of Indonesian maid Erwiana Sulistyaningsih, whose Hong Kong employer received a six-year jail sentence last year.

The new report defined forced labour as employment for which the worker had not been recruited freely, was not doing the job freely, or could not walk away from work.

Fourteen percent of those in forced labour had been trafficked into the city, it said.

"Hong Kong must come clean and acknowledge these problems. It can no longer afford to sweep them under the carpet," said Piya Muqit, executive director of Justice Centre, a non-profit rights group.

"Current regulations can actually increase the vulnerability of workers to exploitation and victims face very real barriers in seeking assistance and justice," she said.

Debt incurred by unscrupulous employment agencies both in Hong Kong and the workers' home countries also played a major role in trapping workers in their jobs, the report found.

"Forced labour does not always involve physical violence, there are many tools of coercion and deception," said Victoria Wisniewski Otero, co-author of the study that interviewed more than 1,000 workers.

One Indonesian maid named as Indah told researchers she felt she had no choice but to continue working because of the debt she had incurred.

She also said she had no access to her passport, which was being held by her employer.

The study found migrant domestic labourers worked an average 70-hour week and more than a third were not given the full 24-hour rest period required under Hong Kong law.

The report called on the Hong Kong government to review legislation, improve workers' living and working conditions, and penalise agencies that overcharge.

Rules that say foreign maids must live with their employers and leave the city within two weeks of terminating an employment contract must also be scrapped, it said.

"The government will have to be pushed and pulled into doing something," said legislator Emily Lau, head of Hong Kong's Democratic Party, who also called for workers' home countries to take action.

Hong Kong's labour department had no immediate response to the report.