The National Union of Metalworkers of SA is severing ties with the ruling alliance comprising the ANC, the Congress of SA Trade Unions (Cosatu) and the South African Communist Party (SACP).
It said on Monday that it will launch a socialist united front as soon as December.
“We decided to break with the alliance and we resolved to form a United Front and explore the possibility for socialism in South Africa,” the Numsa said in a statement.
But Numsa said it did not plan to voluntarily leave Cosatu.
Cosatu last week postponed a vote to expel Numsa, which has been pursuing a left-wing agenda after falling out with President Jacob Zuma over economic and labour policy.
Some Cosatu unions are also upset at Numsa’s attempts to “poach” their members.
Numsa is the biggest union in the country with over 340 000 members.
At a special meeting in December last year, Numsa resolved to withdraw its support for the ANC in the May general elections.