The number of companies requiring workers to put in overtime hours for which they are not fully compensated has surged, reaching a 30-year high, labor officials and news reports said Tuesday.

A total of 17,000 companies, or 13 percent of the total visited by labor officials as part of routine inspections, admitted to failing to pay overtime or nighttime allowances last year, labor ministry spokesman Shuji Kawamata said.

The Nihon Keizai Shimbun, a leading business daily, reported earlier in the day that the number of violations was the highest since 1971.

Kawamata said violations have been on the rise in recent years but could not immediately confirm the newspaper report. The number of violations was just over 7,000 cases five years ago, he said.

Nearly 50 severe cases, up by 15 from a year earlier, were reported to prosecutors for possible criminal investigation, Kawamata said. The violations otherwise generally result in only a warning, with no fine.

Most of the violators were small and medium-size companies hit hard by the prolonged economic slump.

The labor ministry issued guidelines in May urging employers to observe the labor law, which requires an additional 25 percent pay for every hour exceeding an eight-hour day, as well as another 25 percent for overnight work.