Undercover filming by BBC investigative program Panoramashowed exhausted employees who said they had worked 12 to 16 hours a day with no days off for long stretches.
They lived 12 to a room in dormitories and were forced to attend before and after work meetings without being paid for their time, the BBC said.
The BBC program alleges Apple is not living up to its promise to customers to improve working conditions in China.
Apple responded to the allegations with an internal memo in which Apple executive Jeff Williams and CEO Tim Cook said they were "deeply offended by the suggestion that Apple would break a promise to the workers in our supply chain."
Apple promised to improve conditions in its Chinese factories in 2010, after poor conditions were exposed following 14 suicides by workers at its Foxconn supplier in China.
Apple published a set of standards at the time for treatment of its factory workers and shifted some of its production to another factory, Pegatron, outside of Shanghai.
Long shifts, tired workers
The BBC’s investigation focused on the Pegatron employees, filming workers falling asleep while on their 12-hour shifts.
Panorama also investigated Apple’s claim that its materials are ethically sourced. It found some of the tin in Apple’s products came from illegal mines in Indonesia, where child labour was used to dig tin ore out by hand in dangerous conditions.
Apple denied that it has broken its promise to improve working conditions and source materials ethically.
In the internal memo, Apple said the company regularly audits operations in China and has a team of experts there to ensure compliance with its supplier code of conduct.
"Sometimes critics point to the discovery of problems as evidence that the process isn't working," the memo read.
Apple audit system
"The reality is that we find violations in every audit we have ever performed, no matter how sophisticated the company we're auditing. We find problems, we drive improvement, and then we raise the bar."
The memo said work weeks of more than 70 hours were common several years ago, but Apple now has a standard of fewer than 60 hours a week for its suppliers.
The company claimed it is also addressing illegal tin mining by spearheading an Indonesian Tin Working Group with other technology companies that holds smelters accountable for where tin is sourced and how it is mined.
Apple said it resents Panorama’s suggestion that it is not living up to its promise to customers to improve working conditions.
"We know of no other company doing as much as Apple does to ensure fair and safe working conditions, to discover and investigate problems, to fix and follow through when issues arise, and to provide transparency into the operations of our suppliers," Apple said.