Foxconn has acknowledged hiring teenagers as young as 14 in a Chinese factory.
Labour rights activists in China have accused Foxconn and other big employers in China of using student interns as a cheap source of labour for production lines where it is more difficult to attract young adult workers to lower paid jobs.
Foxconn, the trading name of Taiwan's Hon Hai Precision Industry, said it had found some interns at a plant in Yantai, in northeastern Shandong province, were under the legal working age of 16. It did not say how many were underage.
"Our investigation has shown that the interns in question, who ranged in age from 14 to 16, had worked in that campus for approximately three weeks," it said in a statement on Tuesday.
"This is not only a violation of China's labour law, it is also a violation of Foxconn policy and immediate steps have been taken to return the interns in question to their educational institutions," it said.
Foxconn is Apple's largest manufacturing partner, and also makes products for Dell, Sony and Hewlett-Packard among its other clients. It said the Yantai plant does not make Apple products.
Foxconn made the announcement after investigating Chinese media reports of underage interns among its China workforce of 1.2 million. It said it had found no evidence of similar violations at any of its other plants in China.
Foxconn said it would work with local government to bar the schools involved in the Yantai case from the intern programme unless shown to be compliant with labour law and company policy.
"However, we recognize that full responsibility for these violations rests with our company and we have apologized to each of the students for our role in this action," the firm said.
Foxconn and Apple have been forced to improve working conditions at Chinese factories that make most of the world's iPads and iPhones after a series of well-publicised suicides in 2010 and reports of labour abuses, such as excessive overtime, threw a spotlight on conditions inside the plants.
Last month, a riot broke out at a Foxconn plant assembling iPhones in the northern city of Taiyuan over living conditions inside Foxconn's on-site dormitories for migrant workers.
In response to the scrutiny, Foxconn plans to cut overtime to less than nine hours a week from the current 20.
It defended its intern programme on Tuesday, saying they made up only 2.7 percent of its workforce in China. Internships could be long-term or short-term, carried out in cooperation with vocational schools and other educational institutions.
The average internship lasted about three-and-a-half months, it said.