The company, which makes products for Apple and Sony, made the admission following reports by a rights group and mainland media that students on an internship programme at the plant had been under the legal age of 16.

"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 in a statement late Tuesday.

Foxconn, the world's biggest contract manufacturer, said it had carried out a probe at the plant in Shandong province showing that the interns in question, who were aged between 14 and 16, had worked in the plant for about three weeks.

"We have found no evidence of similar violations in any of our other campuses in China but we will not hesitate to take immediate action in any campus if any violations are discovered," it said.

The company issued the statement after Chinese media and US-based China Labor Watch reported on the use of underage workers at the plant.

Foxconn said in its statement it has long had a short-term internship programme carried out in cooperation with vocational schools and other educational institutions in China.

Interns represent approximately 2.7 percent of its workforce of 1.2 million employees in China, it added.

The news is the latest in a string of problems to beset Foxconn, which has frequently been targeted for its labour practices following a spate of apparent suicides in 2010 that activists blamed on tough working conditions, prompting calls for better treatment of staff.

And earlier this month it was hit by two labour disputes that saw thousands of people go on strike, which China Labour Watch said was over increased product quality levels and demands that staff work through a national holiday.

Foxconn is the world's largest maker of computer components and assembles products for Apple and Sony as well as Intel and Nokia, among others.