Apple has been accused of using Chinese suppliers who exhaust their workers, provide inadequate training and impose other harsh working conditions.
A BBC Panorama report into a Pegatron factory in Shanghai, one of Apple's major suppliers, said workers were treated "like members of a production army".
The BBC found that workers hours, ID cards, dorm standards and rules regarding young workers were broken at Pegatron's factories. It said that workers preparing for the launch of the iPhone 6 would regularly work until they fell asleep on the production line after 12 hour shifts.
Another undercover worker, who had to endure a 16 hour shift, said: "Every time I got back to the dormitories, I wouldn't want to move. Even if I was hungry I wouldn't want to get up to eat. I just wanted to lie down and rest. I was unable to sleep at night because of the stress."
The BBC showed undercover footage of workers taking a training "exam" in which answers were chanted out by large groups of workers.
The BBC also traveled to the Indonesian island of Bangka, where it says children as young as 12 are involved in gathering tin ore, used by tech firms the world over for almost every kind of product. Apple says it does gather tin from Bangka, but not from illegal sites.
Apple told the BBC it disagreed with the programme's conclusions. It reported that it monitored working conditions for more than 1 million Pegatron employees' working hours, which it said averaged to 55 hours per week -- within its guidelines.
Apple also has a number of watchdogs and auditors in place to monitor working conditions, some of which were introduced following previous controversies such as the suicide of 14 workers at a Foxconn plant in 2010.
Though it was not interviewed for the programme, Apple said in a statement:
"We are aware of no other company doing as much as Apple to ensure fair and safe working conditions.
"We work with suppliers to address shortfalls, and we see continuous and significant improvement, but we know our work is never done."
On the issue of Indonesian tin, Apple said that it would continue to buy from local suppliers in a effort to improve conditions rather than pull out. It told the BBC:
"The simplest course of action would be for Apple to unilaterally refuse any tin from Indonesian mines. That would be easy for us to do and would certainly shield us from criticism. But that would also be the lazy and cowardly path, since it would do nothing to improve the situation. We have chosen to stay engaged and attempt to drive changes on the ground."
Apple's published standards of conduct from suppliers can be read here.
In a separate statement, Pegatron said:
"Worker safety and well-being are our top priorities. We set very high standards, conduct rigorous training for managers and workers, and have external auditors regularly visiting our facilities to find areas for improvement."