A labour watchdog on Wednesday slammed the London Olympics organisers over alleged human rights abuses at Chinese "sweatshops" producing Games merchandise.

The group said labourers at two Chinese factories producing merchandise including Olympics mascots Wenlock and Mandeville worked up to 120 hours of overtime a month, or nearly three times the legal limit.

The workers were exposed to hazardous chemicals without sufficient protective gear, leading to illnesses, and some had to buy their own face masks to guard against paint mist.

"We are disappointed as these practices are unacceptable," Hong Kong-based Students and Scholars Against Corporate Misbehaviour (Sacom) spokeswoman Debby Chan told AFP.

Sacom said it based its findings on interviews with 90 workers at two factories in southern Guangdong province between May and June.

The factories were run by Hong Kong companies Key Pine and Zindart Manufacturing.

Some of the workers reported inhaling paint, being docked half a day's wages if they were five minutes late, and being required to start shifts at 8:00 am after finishing at midnight the previous day.

Sacom said the conditions breached the London Organising Committee's (Locog) ethical and sustainable procurement codes.

"The rampant rights violations reveal that the Locog codes are really no more than lip service with no commitment to the enforcement of labour rights standards," the group said in a report.

Sacom, which has previously highlighted conditions of workers producing Apple products in China, urged Logoc to investigate the Chinese factories.

It also demanded the International Olympic Committee (IOC) prevent such labour abuses in the future.

Zindart told AFP it was investigating the claims, while a representative at Key Pine said the company was not aware of the report. Both declined further comment.

Foreign firms have increasingly turned to China for its cheap labour, but rights workers say labour abuses are widespread despite the government's pledges to improve conditions.

New York-based China Labor Watch last month said an investigation of 10 suppliers to Apple in southern and eastern China uncovered violations of workers' rights, including excessive overtime and dangerous conditions.

In March the Fair Labor Association also reported forced overtime and other problems at three of Apple's Chinese suppliers.