SEJONG, South Korea, May 20 (Reuters) - Samsung Electronics' management and its labour union failed to reach a deal on Wednesday, setting the stage for 48,000 workers to walk off the job on Thursday, threatening the health of South Korea's economy and likely disrupting the global supply of semiconductors.

Under intense pressure from the government and business groups to avert a strike, the two sides had sought to hash out a deal on bonus payments before the 18-day strike begins.

Samsung said in a statement that the union had made unacceptable demands and that accepting them would have shaken the company's fundamental principles.

The union demanded that Samsung abolish a cap on bonuses that stands at 50% of annual salaries, allocate 15% of annual operating profit to bonuses and that these changes be formalised beyond one year.