Apple’s main supplier in Asia has been employing students illegally working overtime to assemble the iPhone X, as it struggles to catch up with demand after production delays.

Six high school students told the Financial Times they routinely work 11-hour days assembling the iPhone X at a factory in Zhengzhou, China, which constitutes illegal overtime for student interns under Chinese law.

The six said they were among a group of 3,000 students from Zhengzhou Urban Rail Transit School sent in September to work at the local facility run by Taiwan-based Apple supplier Hon Hai Precision Industry, better known as Foxconn.

The students, aged 17 to 19, said they were told that a three-month stint at the factory was required “work experience” that they had to complete in order to graduate.

“We are being forced by our school to work here,” said Ms Yang, an 18-year-old student training to be a train attendant who declined to use her first name for fear of punishment. “The work has nothing to do with our studies.” She said she assembled up to 1,200 iPhone X cameras a day.

The school declined to comment.

When contacted about the students’ complaints, Apple and Foxconn acknowledged they had discovered cases of student interns working overtime and said they were taking remedial action. But both companies said the students were working voluntarily.

Apple said an audit has turned up “instances of student interns working overtime at a supplier facility in China”, adding “we’ve confirmed the students worked voluntarily, were compensated and provided benefits, but they should not have been allowed to work overtime”.

Foxconn said that “all work was voluntary and compensated appropriately, [but] the interns did work overtime in violation of our policy” prohibiting student interns working more than 40 hours a week.

The launch of the anniversary iPhone X was marred by production issues and was delayed to November from Apple’s typical September release date. The weeks of idle capacity caused Foxconn’s quarterly profit to drop 39 per cent.

According to a long-time Foxconn employee, the Zhengzhou factory hires students every year during the busy season between August and December. Such hiring can swell numbers at the plant from a base of 100,000 to more than 300,000 workers producing up to 20,000 iPhones a day, the employee said.

But this year, the need for seasonal workers was greater, the employee added.

“The purchasing practices of Apple and others are designed to cut costs, and do things ‘just in time’,” said Jenny Chan, assistant professor at the Hong Kong Polytechnic University. “This leads to the use of student labourers who can be flexibly hired.”

Foxconn said its internship programme was “carried out in co-operation with local governments and a number of vocational schools in China”.

The education ministry of Henan, where Zhengzhou is the capital, issued notices to all vocational schools in the central Chinese province to send their “work experience students” to Foxconn, according to a person who saw the notice.

Students also came from the nearby cities of Kaifeng, Nanyang, and Xinxiang, according to a Foxconn employee working on the iPhone X.

The education ministry could not be reached for comment.

Providing flexible student labour is one of the preferential policies that the Henan government offers Foxconn in order to keep it there.

“Henan province does everything in its power to make sure Foxconn thrives here. It’s not just about tax revenues — it’s about industrial upgrading, building an export industry, having a foreign partnership [with Apple],” said a local official.