LONDON — A simmering dispute within South Africa’s political establishment moved closer to a showdown on Monday when the country’s biggest labor union announced that it would break with the governing African Nation Congress and form a new socialist political party.

The move by the National Union of Metalworkers of South Africa, known as Numsa, dealt a significant blow to the coalition of labor and political forces that has dominated the country since the formal end of apartheid in 1994. It threatened to speed the realignment of political forces in South Africa and strengthen the government’s adversaries on the left.

But, as the country’s leaders and factions maneuvered on Monday, the full impact of Numsa’s step remained unclear.

“We decided to break with the alliance, and we resolved to form a united front and explore the possibility for socialism in South Africa,” Numsa said in a statement, quoted in news reports, that gave no further details.