A company making iPads and iPhones is condemned for allegedly employing 14-year-olds and breaking other Chinese labour laws.
Tech giant Apple is under renewed fire over workers' rights in China, according to a report issued by a human rights charity.
China Labor Watch (CLW) said it has documented violation of work laws, forced excessive overtime and underage employees at Pegatron, where Apple's iPads and iPhones are made.
The abuses are alleged to have taken place at facilities owned by the Taiwan-based manufacturer, which is subcontracted to make Apple gadgets.
New York-based CLW said workers' rights were violated at several of Pegatron's factories in Shanghai and Suzhou.
The report said many workers were students or teens, with some forced to work standing up for as long as 11 hours.
Up to 12 workers shared cramped dormitories with rudimentary facilities.
"The Pegatron factories are violating a great number of international and Chinese laws and standards as well as the standards of Apple's own social responsibility code of conduct," CLW said in the report.
Pegatron, which has market capitalisation of around £500m, said in a statement that it would investigate the matter and would take immediate action to correct any violations of Chinese labour laws and its own code of conduct.
"We strive to make each day at Pegatron better than the last for our employees. They are the heart of our business," Pegatron's CEO Jason Cheng said in the statement.
"That's why we take these allegations very seriously."
Pegatron posted revenues of around £4.2bn for the first quarter of 2013, up 30.9% on the same period last year, due primarily to tablet growth.
Apple, responding to the CLW report, said it had conducted 15 audits at Pegatron facilities since 2007 that covered more than 130,000 workers to ensure safe and fair working conditions throughout its supply chain.
It has been in touch with CLW for several months and has fixed some issues raised by the organisation, Apple said.
"Their latest report contains claims that are new to us and we will investigate them immediately," Apple said.
"If our audits find that workers have been underpaid or denied compensation for any time they've worked, we will require that Pegatron reimburse them in full."
CLW said it sent undercover investigators into three Pegatron factories and conducted nearly 200 interviews with workers outside the factories from March to July.
It said it discovered 86 violations at the three factories making Apple products.
Pegatron's factories in China now employ more than 70,000 workers after it stepped up production of Apple's products as part of the US technology giant's plans to diversify its contract manufacturing partners.
Foxconn Technology Group, which has also been criticised by labour groups for poor working conditions, suicide rates and underage staff, now makes most of Apple's top products through its flagship unit, Hon Hai Precision Industry.