More than 80 percent of companies that employed truck, bus and taxi drivers last year breached the Labor Standards Law, with illegal overtime accounting for over half the violations, a government survey says.

Of the 5,436 supervised firms surveyed, breaches were found at 4,564, or 84 percent, the labor ministry survey found, adding that 61 committed especially serious violations of the labor law and other regulations in 2017 that resulted in their referral to prosecutors.

Automobile drivers will be exempted for five years from a legal overtime cap to be introduced at major firms next April, but the results of the survey revealed that many are forced to work long hours.

Of the violators, 4,564, or 58.2 percent, contravened labor time regulations, followed by 21.5 percent that violated rules on extra payment.

The survey also found that some truck drivers were on duty for about 320 hours per month, exceeding the basic upper limit of 293 hours set by the government.

The ministry also found that some expressway bus companies made drivers break the overtime caps set in their working agreements.

The labor law requires employers to reach written agreements with workers and submit them to the relevant authorities before prescribing overtime in excess of an eight-hour working day or a 40-hour work week.

Professional drivers have been facing tougher working conditions on the back of intensifying price competition and an acute labor shortage.

In January 2016, a ski tour bus from Tokyo careened off a road in the resort town of Karuizawa, Nagano Prefecture, killing 13 passengers and two drivers.

The Tokyo-based operator, ESP, was indicted Friday for illegally making some of its drivers work overtime without drafting an agreement.