Nearly 1 in 4 companies have admitted that some employees do more than 80 hours of overwork per month, according to the nation's first white paper released Friday on karoshi, or death by overwork.

According to the survey, taken between last December and January this year, about 10.8 percent of companies said they had workers putting in 80 to 100 hours of overtime a month, while another 11.9 percent said they had workers doing more than 100 hours.

The paper, said to be the first of its kind in the world, was based on responses from 1,743 companies and 19,583 workers. It had targeted 10,000 firms and 20,000 employees.