The National Union of Metalworkers of South Africa (Numsa) announced the break at a special congress for members.

“Numsa as an organisation will neither endorse nor support the ANC or any other party in 2014,” said leader Irvin Jim.

With more than 300,000 members, Numsa is an influential source of votes, cash and grassroots organisation in South African politics.

The ANC’s dominant electoral position has been built upon a “tripartite” alliance with the trade union umbrella group Cosatu — of which Numsa is the biggest member — and the Communist Party.

The ANC has won every election since 1994 — when the late Nelson Mandela became South Africa’s first black president — but it has come under fierce criticism amid high unemployment and the unpopularity of current President Jacob Zuma.

Numsa’s leadership has recently expressed anger at the government’s centrist economic policies and the shooting dead of 34 striking miners by police last year.

Jim also floated the idea of establishing a new socialist party.

“Numsa will explore the establishment of a Movement for Socialism as the working class needs a political organisation committed to its policies and actions.”