SEOUL (Reuters) - Hyundai Motor's unionised workers in South Korea plan to take strike action for a fifth year in a row after annual wage talks broke down, a spokesman for the union said on Thursday.

A total of 77 percent of Hyundai's 48,806 unionised workers in South Korea, or 85 percent of those of who voted, approved the potential strike action.

"The overwhelming support is a natural result of the anger of our 48,000 union workers," the union said in a statement.

The prospect of a walkout comes as Hyundai Motor, the world's fifth-biggest carmaker together with affiliate Kia Motors, posted a 0.9 percent fall in sales in the first half, dragged lower by an emerging market slowdown.

A prolonged strike could hurt sales of high-demand cars such as Hyundai's Tucson sport utility vehicle and add to troubles for an automaker already struggling with slowing sales of its staple sedans, analysts said.

Union negotiators led by hardline boss Park You-ki will meet on Thursday to decide when to stage the walkout.

Hyundai Motor has been hit by strikes in all but four of the union's 29-year history. The automaker usually makes up for lost production later each year.

This year, the union is demanding a 7.2 percent rise in the basic monthly wage and performance pay totaling 30 percent of the automaker's 2015 net profit.

Other demands include giving employees the right to refuse to be promoted so that they can maintain their union membership.

The company is pushing to freeze wages, revamp the wage structure and expand the so-called "peak-wage system," the union said.

(Reporting by Hyunjoo Jin; editing by David Clarke)