South Africa’s biggest union said on Monday it was breaking its alliance with the ruling African National Congress (ANC) and launching a socialist party, in a major blow to a coalition that has governed since apartheid ended in 1994.

“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 National Union of Metalworkers of South Africa (NUMSA) said in a statement.

•Zuma: NUMSA divorces ANC

But NUMSA said it did not plan to voluntarily leave the COSATU labour federation that is the ANC’s official partner in a governing alliance forged in the struggle against apartheid that also includes the South African Communist Party.

COSATU last week postponed a vote to expel NUMSA, which has been pursuing a left-wing agenda after falling out with President Jacob Zuma’s ANC over economic and labour policy.

Some COSATU unions are also upset at NUMSA’s attempts to “poach” their members. NUMSA, which says it has 340,000 members, did not support the ANC in general elections the party won comfortably earlier this year.

Most of NUMSA’s members are black workers in key sectors such as car manufacturing. The ANC is keen to see it remain in COSATU and regain its support.