A woman in Japan died from overwork after logging 159 hours of overtime in the month leading up to her death, labour inspectors have ruled.

Miwa Sado, a political journalist at the country’s national broadcaster, suffered heart failure in July 2013, though her employer only made the case public this week.

Officials in Tokyo deemed the 31-year-old had died from karoshi – death from overwork – after taking just two days off in the 30 days before she died, reports The Japan Times.

As a journalist for NHK, Ms Sado covered the Tokyo Metropolitan Government, dying just three days after reporting on a local election in the House of Councillors.

Masahiko Yamauchi, a senior official at the broadcaster, said Ms Sado’s death was a “problem for our organisation as a whole, including the labour system and how elections are covered”.

In a comment released by NHK, her parents said: “Even today, four years after, we cannot accept our daughter’s death as a reality.

“We hope that the sorrow of the bereaved family will never be wasted.”

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In 2015 the suicide of an advertising agency employee, who was working 100 hours overtime a month, prompted calls for a shift in Japan’s work culture.

According to a national survey, a fifth of the country's workforce are at risk of karoshi, since they clock more than 80 hours extra work time each month.

With around 2,000 people a year killing themselves due to work-related stress, the Japanese government has recently taken measures to address the issue.