The Japanese government's first investigation into the problem of karoshi, or death from overwork, has revealed that staff at 12 per cent of companies put in more than 100 hours of overtime every month.

Conditions are only marginally better at 23 per cent of the firms that were questioned for the White Paper, where 80 hours of overtime a month are the norm.

And the real figures may be even more shocking as only 1,743 of the 10,000 companies that were asked to take part in the survey complied. Of the 20,000 workers invited to provide information for the study, 19,583 replied in full.

In the year that ended March 31, the health ministry identified 93 suicides and attempted suicides as being caused by overwork. Police statistics, however, claim there were 2,159 suicides that could be attributed to problems related to work.

Government statistics also show that legal cases filed over karoshi soared to 1,456 in 12-month period that ended in March 2015. In comparison, a total of 1,576 cases were filed between 2004 and 2008.