Investigators have discovered 7-Eleven Japan to have been underpaying staff since the 1970s, according to a Nikkei report.

The review uncovered inaccuracies in the formula used by the retailer to calculate overtime wages for part-time staff since the business was established. The error resulted in staff earning less than mandated by Japan’s Labour Standards Act. All staff payment calculations are performed by the 7-Eleven Japan’s head office, which guide payment levels for franchise operators. 

The company, which was first alerted to the problem in October, has responded by promising to compensate around 30,000 affected staff for underpayments going back as far as the company has kept its archives since March 2012. Staff seeking compensation for underpayments dating from before that time can be paid if they have preserved records.

7-Eleven Japan could face a bill for compensatory payments of ¥490 million (US$4.51 million).