The world’s biggest laptop brands rely on Chinese student labourers as young as 16 to work 12-hour days on factory production lines, with their funding and graduation at risk if they fail to comply, a labour watchdog has claimed.

Students on vocational courses are recruited in their thousands to produce keyboards, assemble parts and fit screws during internships at Quanta Computer, a Taiwan-owned Fortune 500 company whose factories in China supply Apple, Acer, Hewlett-Packard and Sony, among many others.

If students refuse, they are told their funding will be cut or they will not receive their diploma, claims a report by Students and Scholars Against Corporate Misbehaviour (Sacom), a Hong Kong-based non-profit.

Interns are believed to comprise about half of the workforce at Quanta’s Chongqing factory, where students’ three-month internships supply a steady flow of “cheap and disposable labour”, the report alleges. Quanta operates three manufacturing plants in mainland China, in Shanghai, Changshu and Chongqing, from which it claims to supply each of the top 10 PC companies and produce one-third of all PC laptops in the world.

“We were forced to come,” said one 18-year-old intern at Quanta Chongqing, where students were assembling parts for Acer, HP and Sony laptops and tablets, according to Sacom. “Every semester, our school recruits new students but our campus is small. When they don’t have enough space in the classrooms or dormitories, they force students out to do internships and then let the new students stay in our dorms.”

Another 16-year-old intern said: “If we refused, we would not be able to get our graduation certificates. Also, our dining and accommodation subsidies would be cancelled.”

Undercover research and interviews at Quanta’s Chongqing factory, which took place in late 2016 and mid-2017, alleged that interns were working 12-hour days for months at a time without a day off. Students had also been hired illegally through recruitment agencies and paid illegal deposits to the recruitment agencies or their own universities to “secure” the internship, said Sacom.

Despite labour laws stipulating that interns should make up just 10% of a facility’s workforce, a manager at Quanta Chongqing told undercover researchers that he estimated “more than 60%” of the workers were interns.

“Quanta is cooperating with local vocational schools to arrange student interns to work in the factory,” the manager said. “Student interns are good because they are flexible. It only takes a few weeks to order the students from the schools.”

In an email to the Guardian, Quanta Computer denied the report’s allegations and said: “After internal verification, we believe that the allegations … are untrue and unfair to the company. There are serious mistakes in the information … from the ‘undercover investigators’.”

Quanta Computer did not specify what those mistakes may be, and did not respond to further requests for comment.

Acer and Sony did not respond to requests for comment. HP told the Guardian: “HP is committed to the highest standards of business conduct and maintains robust compliance programs to ensure our suppliers adhere to HP’s standards everywhere in the world. HP regularly monitors the performance of its partners to review their compliance. Failure to comply will result in appropriate action, including termination.”

Apple told the Guardian that “it does not have any operations at the Quanta Chongqing facility”.

The phenomenon of hiring students first came to media attention in 2012, when ot was alleged that students on forced internships at Foxconn were assembling iPhones. Despite improved Chinese labour laws intended to stem the practice, interns are still hired to provide a cheap, quick fix to labour shortages, and to benefit the needs of Chinese universities, said Sacom’s project officer Michael Ma.

“Student internships are difficult to terminate, because their brief contracts fit volatile labour needs in the electronic industry. And vocational schools can escape their responsibilities of arranging classes and providing dormitories by sending students to factories,” said Ma.

“Even though Sacom has been addressing these issues since 2012, the situation is getting worse because of [profit and labour] competition between multinationals and the lack of trade unions in China, which prevents workers from taking action against such practices.”

Chinese law requires internships to be directly related to students’ course of study. Of nine students interviewed by Sacom, only four were studying electronics. The others had majors ranging from accounting to early education, hotel management, costume design, and automobile repair.

Even electrical engineering students said work on the production lines was unrelated to their studies.

“Yes, I do study electronics, but what can I learn from merely inserting screws all day for three months?” one intern said.

“I learned nothing here,” said an intern studying costume design. “After leaving Quanta we’ll be sent to a factory for repairing automobiles.”

A year after the 2012 claims about students on forced internships at Foxconn, it was alleged that students were being forced to assemble Sony’s Playstation 4, or risk failing their course. Further allegations came in 2015, an investigation linked Dell and HP to forced internships at Wistron, a Chinese electronics manufacturer.

Jenny Chan, assistant professor at Hong Kong Polytechnical University and researcher of student labour in China, describes students as “overworked and unguided” during their “widespread, ‘sheep-like’ internships”.

Chongqing, a sprawling 30m megacity, is known as the “land of laptops”. One-third of the entire city’s exports is now in notebook computers, and over half of the world’s top 500 companies operate out of the city.

But researchers say weaker law enforcement and labour monitoring by civil society in inland China – compared to coastal China, from where many electronics companies have relocated – means that labour abuses are able to go largely unchecked. Inland wages are also lower and local government tax allowances to attract businesses bigger, according to Sacom.