A fifth of the Japanese workforce are at risk of death from overwork, says a new government survey.
Hundreds of deaths related to overwork including from strokes, heart attacks and suicides are reported every year in Japan.
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 1743 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 2159 suicides that could be attributed to problems related to work.
Government statistics also show that legal cases filed over karoshi soared to 1456 in 12-month period that ended in March 2015. In comparison, a total of 1576 cases were filed between 2004 and 2008.
Nevertheless, campaigners against companies that force employees to put in long hours, frequently unpaid, have welcomed the report as a positive first step.
"I would like to commend the concrete action taken by compiling a white paper," Emiko Teranishi, head of the Families Dealing with Karoshi support group, told the Mainichi newspaper.
"I would like to see more detailed studies on the backgrounds of individual cases, leading to specific measures," said Teranishi, whose own husband committed suicide after being forced to work long hours.
Legislation went into effect in November 2014 that requires the national Government to carry out measures to prevent suicides and deaths form overwork, although the rules are hampered by a lack of penalties for companies that fail to comply.
Statistics compiled by the Organisation for Economic Co-operation and Development show that the average Japanese works 33.25 hours a week, one hour more than the average British worker. But those figures fail to include "service overtime," the unpaid extra hours that at 12 per cent of Japanese firms are adding 100 hours of work every month.
Service overtime has long been accepted practice at Japanese corporations but has become more common since the nation's economic bubble burst in the early 1990s and employees increasingly feared for their futures.
Unions have similarly been complicit in the problem, signing agreements with companies that waive the legally recognised limits on hours that employees are permitted to work.