Apple Computer has said a report of labor conditions at its iPod plant in China found workers did more than 60 hours a week a third of the time.
Staff making the world’s most popular MP3 player also worked more than six consecutive days 25% of the time.
Apple said the hours were "excessive" and said its supplier would now be enforcing a "normal" 60-hour week.
The California-based firm said its report found "no evidence of enforced labour" or use of child workers.
The computer firm sent an audit team to its unnamed plant in China after a British newspaper published a story alleging poor working practices.
The supplier of iPods has to follow a code of conduct laid down by Apple.
In a statement Apple said: "Our investigation found that our top iPod manufacturing partner, Foxconn, complies with our supplier code of conduct in most areas and is taking steps to correct the violations we found."
But the report has been criticised by a leading international trade union organisation for not being independently verified.
Janek Kuczkiewicz, director of human and trade union rights at the International Confederation of Free Trade Unions (ICFTU), said he was not impressed by the report.
Apple said its team interviewed 100 workers at the plant, cross-checked staff logs and visited factory floors and dormitories.
The plant supports 200,000 employees, less than 15% of which work on making the iPod, and 32,000 staff live on-site at the plant.
Mr Kuczkiewicz said: "We are not impressed either by the report or by the findings of Apple."
"Apple interviewed just 100 people out of the estimated 30,000 iPod workers.
"We do not know the conditions in which the interviews were held. We have serious reservations about the report."
The audit team said staff earned "at least the local minimum wage" and that half of the 100 people it interviewed earned above that amount.
We have serious reservations about the report
Janek Kuczkiewicz, ICFTU
Apple did not specify what the minimum wage for the area was but the original report in the Mail on Sunday said staff earned as little as £27 a month.
The computer firm announced revenue of $4.37bn in its last quarterly statement, selling eight million iPods in the last three months.
"[The] iPod continued to earn a US market share of over 75% and we are extremely excited about future iPod products in our pipeline," said Steve Jobs, the firm’s chief executive, at the time the financial statements were released..
Apple recently announced almost $5bn in revenue for its last quarter
Employees at the iPod plant can also earn bonuses but Apple said the pay structure was "unnecessarily complex" and that its supplier would be introducing a simpler system.
"We found no instances of forced overtime and employees confirmed in interviews that they could decline overtime requests without penalty," said Apple in a statement.
The firm said there were "overtime limit exceptions in unusual circumstances" and that it supported a healthy work-life balance.
But it did not specify what the triggers for "unusual circumstances" were and what upper limit it set on working hours.
Mr Kuczkiewicz said Apple had not asked workers what they preferred – a decent wage or minimum wage and overtime.
"We believe it is the workers’ role to monitor standards. That has not happened at the Apple plant in China.
"We would like to remind Apple there are other labour standards – freedom from discrimination, freedom of association and freedom to bargain collectively."
The company discovered two instances of staff being made to stand to attention as a form of disciplinary punishment.
"While we did not find this practice to be widespread, Apple has a zero tolerance policy for any instance, isolated or not, of any treatment of workers that could be interpreted as harsh," the company said.
The supplier has started an "aggressive manager and employee training program" to ensure this is not repeated, said Apple.
The audit team also reported:
- Employees work in "factories that are generally bright, clean and modern with air-conditioned assembly line areas, and are provided with protective gear" .
- The process for reporting overtime was "manual and monthly, and while not a violation of the Code of Conduct, it was subject to human error and relied too much on memory for dispute resolution".
- The campus includes "factories, employee housing, banks, a post office, a hospital, supermarkets, and a variety of recreational facilities including soccer fields, a swimming pool, TV lounges and internet cafes".
- Dormitories found did not violate its code of conduct but "we were not satisfied, however, with the living conditions of three of the off-site leased dorms that we visited".
Apple said it had hired a workplace standards firm to work with the firm and had joined the Electronic Industry Code of Compliance Implementation Group.
"We are dedicated to ensuring that working conditions are safe and employees are treated with respect and dignity wherever Apple products are made," the firm said.