Nova Corp., Japan's largest employer of foreign nationals, is set to amend its working times from this week, a move which could see thousands of foreign teachers become ineligible for Employees Health Insurance, and save the company as much as 1 billion yen a year in the process.

Teachers representatives have slammed the moves, accusing Nova of trying to "wriggle out" of its insurance obligations and placing instructors' security at risk.

The nonenrollment of full-time workers in the Employees Health Insurance and Pension ("shakai hoken") system is illegal in Japan, where companies must enroll all employees working 75 percent of the hours of a full-timer, regardless of their nationality. The cost of the insurance premiums is shared equally between employer and employee.