The walkout, which was approved last week by union representatives, began as scheduled Monday afternoon, said Jung Jun-yung, the head of the union's overseas co-operation department. Unionized workers plan to lay down tools Monday for a total of eight hours at three different factories.

Hyundai Motor confirmed the strike was underway, company spokesman Jake Jang said. Hyundai asked the Ulsan District Court to issue an order barring the walkout, Jang said, though adding no decision had yet been reached.

Ulsan is an industrial city about 400 kilometres southeast of Seoul where the world's sixth-largest automaker has its main factory.

Labour troubles are a near constant headache for Hyundai. The company's union has gone on strike every year but one since it was established in 1987.

Under the strike plan, workers will carry out normal shifts Tuesday, but expand the strike Wednesday to a total of 12 hours at each of the three factories. They will decide further action, if any, on Wednesday.

A total of 400 representatives voted unanimously Friday for the 44,000-strong member union to walk off the job.

Unionized workers have refused to work overtime since Dec. 28 after workers received a bonus equal to one month's salary, which they say is less than the agreed amount of a payment equal to 1.5 months.

The company says the latter total was based on an incentive to reach the company's 2006 production target and since that was not achieved because of strikes the payment was reduced.

``There would never have been a strike if the labour-management agreement had been realized,'' the union said on its website.

A week ago, Hyundai sued the union, seeking damages to help cover production losses caused by the refusal to do overtime.

Last year was Hyundai's worst ever in terms of strikes. A total of four walkouts cost Hyundai 118,293 vehicles in lost production costing 1.64 trillion won (US$1.75 billion), according to the company.

Hyundai was also dogged by turmoil following the April arrest and jailing of chairman Chung Mong-koo on embezzlement and breach of trust charges related to a slush fund scandal.

Chung, 68, was released from detention in late June on bail and returned to work after spending time in a hospital. He is currently on trial and was due to appear in court on Tuesday.