An extensive investigation by The New York Times claims that Apple has knowingly overlooked labour violations by its Chinese suppliers as it seeks to meet demand for its products.

The Times report is based on the testimony of “three dozen current or former employees and contractors”, six of which are anonymous Apple executives with direct experience of the company’s supplier responsibility group.

According to both company reports and “advocacy groups that, within China, are often considered reliable, independent monitors,” the workers assembling iPhones, iPads and other devices work excessive overtime and live in crowded dorms. Under-age workers are employed, records are falsified, and hazardous waste is disposed of improperly.

Foxconn builds Apple’s products. The manufacturer also assembles hardware for Microsoft and Nintendo.

“We've known about labour abuses in some factories for four years, and they're still going on,” said one former Apple executive. “Why? Because the system works for us. Suppliers would change everything tomorrow if Apple told them they didn't have another choice.”

“If half of iPhones were malfunctioning, do you think Apple would let it go on for four years?”

In one incident, claim the sources, Apple was warned of hazardous working conditions at a plant in Chengdu. An explosion at the factory, and a second at another location, together killed four workers and injured a further 77.

“Apple never cared about anything other than increasing product quality and decreasing production cost,” said Former Foxconn executive Li Mingqi. Mr. Li helped to manage the Chengdu factory at the time of the explosion. He is currently suing Foxconn over his dismissal. “Workers’ welfare has nothing to do with their interests.”

Former Apple executives claim that the company wishes to see improvements in working conditions at its suppliers, but astronomical demand for its products is causing an “unresolved tension”, reports The Times.

This week, Apple reported quarterly profits of US $13.06 billion on US $46.3 billion in revenue.

“If Apple was warned, and didn’t act, that’s reprehensible,” said Nicholas Ashford of the Chengdu explosion. Mr. Ashford is a former chairman of the National Advisory Committee on Occupational Safety and Health, a group that consults the United States Labor Department. “But what’s morally repugnant in one country is accepted business practices in another, and companies take advantage of that.”