Foxconn, Apple's main supplier in China, has admitted it employed interns as young as 14 years old.
In a statement, the Taiwan-based manufacturer acknowledged that some students who took part in its summer internship programme were below China's minimum legal working age of 16.
The company said the interns were employed at its factory in the city of Yantai in eastern China.
Foxconn has previously been accused of poor conditions for its workers.
The firm is best known for producing iPhones and iPads for Apple, but also makes products for other companies, such as Microsoft and Hewlett-Packard.
"We recognise that full responsibility for these violations rests with our company and we have apologised to each of the students for our role in this action," the electronics manufacturer said in a statement.
"Any Foxconn employee found, through our investigation, to be responsible for these violations will have their employment immediately terminated."
The company said it ordered an internal investigation after Chinese media reports and a US-based campaign group China Labor Watch had said it was hiring under-age interns.
The firm said it was trying to determine how the interns ended up at the factory.
China Labor Watch said in a statement: "The under-age interns were mainly sent to Foxconn by schools, but Foxconn did not check the IDs of these young interns.
"The schools involved in this incident should take [main responsibility], but Foxconn is also culpable for not confirming the ages of their workers.
"China Labor Watch calls on the Chinese government to improve the current intern system of Chinese schools."
Fake IDs were only part of the problem said Duncan Innes-Ker, regional editor for China at the Economist Intelligence Unit.
"The market for relatively unskilled labour in China has been tightening, and now there is even more incentive for companies to relax their standards, so they try and fill up their work spaces," he said.
"Companies do have a responsibility to check with the people that they are hiring are above that age.
"The difficulty is the enforcement of regulation which in China has been a grey area for a number of years."
But for companies such as Foxconn that are associated with foreign giants, reputation is important.
That is why the Taiwanese firm has been trying recently to improve its standards.
"Foxconn produces goods for a lot of different companies, and there's been no direct linkage in this particular case to producing Apple products," said Mr Innes-Ker.
"But I think Apple certainly has been putting more pressure on Foxconn to raise its standards, because whenever a story comes out that involves Foxconn, it's inevitable that Apple gets linked with it.
"That's one of the reasons we're seeing more news coming out about Foxconn working towards this goal, improving its labour standards."