Workplace abuses were uncovered in an audit that equated to "a full body scan" of three Chinese factories pumping out coveted Apple gadgets, independent investigators reported.
Forced overtime at factories led to work weeks breaking the limit of 76 hours set by Chinese law, according to the Fair Labor Association (FLA).
In some cased, employees worked more than seven days straight without a required 24-hour break, according to the FLA.
"Overtime. Overtime... That's the key issue to address," FLA president Auret van Heerden said while discussing the findings in an interview with ABC news program Nightline.
"Secondly the exclusion of workers from -- or the failure to integrate workers into all of the committee processes."
Employees should have more involvement with union, health and safety committees, according to van Heerden, who put Foxconn factories on an overall par with modern manufacturing plants elsewhere in the world.
The FLA said that it gave Apple's largest supplier "the equivalent of a full-body scan through 3,000 staff hours investigating three of its factories and surveying more than 35,000 workers."
"Apple and its supplier Foxconn have agreed to our prescriptions, and we will verify progress and report publicly."
Along with excessive overtime and not always compensating workers properly for extra hours, the nearly month-long investigation uncovered health and safety risks and "crucial communication gaps."
Foxconn has pledged to bring factory conditions into full compliance with Chinese law and FLA standards regarding working hours by July of next year, according to the report.
"If implemented, these commitments will significantly improve the lives of more than 1.2 million Foxconn employees and set a new standard for Chinese factories," van Heerden said.
The report was released as Apple chief Tim Cook paid a visit to China, where state media said the man tipped to be country's next leader had told him foreign firms should do more to protect workers.
International labor watchdog groups have said workers in Chinese plants run by major Apple supplier Foxconn of Taiwan are poorly treated, and have blamed a string of apparent suicides on the conditions.
Vice Premier Li Keqiang, who is likely to be China's next premier, met the new Apple chief executive on his visit to Beijing on Tuesday, the official Xinhua news agency reported.
Li told Cook multinational companies should "pay more attention to caring for workers," the report said.
Cook on Wednesday visited a Foxconn plant in China's central city of Zhengzhou, where he viewed the production line, Apple said.
California-based Apple is wildly popular in China, where its products such as the iPhone and iPad are coveted by wealthy consumers.
Apple agreed in January to allow inspections by the FLA following reports that employees were overworked and underpaid at Foxconn factories in China.
Besides Foxconn plants, FLA teams will inspect factories owned by two other Taiwan-owned manufacturers, Quanta and Pegatron, which also make Apple products.
"When completed, the FLA's assessment will cover facilities where more than 90 percent of Apple products are assembled," Apple said earlier this year.
Apple stock price slid slightly after release of the report, inching down to $607 in after-hours trading that followed close of the market in New York.