South Africa's biggest union will not support the ruling ANC in elections next year, its general secretary said on Friday, in a blow to President Jacob Zuma, whose political support with the working class is fast eroding.

The salvo from the 330,000-member National Union of Metalworkers of South Africa (NUMSA) is also another sign that an alliance forged with the African National Congress (ANC) in the common struggle against apartheid is falling apart.

NUMSA is the biggest block in the COSATU labour grouping that is itself part of a formal three-way governing alliance with the ANC that also includes the Communist Party (SACP).

However, NUMSA has been at odds with all three, accusing them of pursuing pro-business policies.

"NUMSA as an organisation will neither endorse nor support the ANC or any other political party in 2014," General Secretary Irvin Jim told a news conference at the end of a meeting of its members.

He said NUMSA officials or workers could campaign for the ANC but would have to do this "in their own time and using their own resources".

"It is clear that the working class cannot any longer see the ANC or the SACP as its class allies in any meaningful sense," Jim said.

He also called on Zuma, who was booed at a memorial for anti-apartheid legend Nelson Mandela on December 10, to resign.

Union support helped propel Zuma to power in 2009 but the relationship has soured since police shot dead 34 striking miners at Lonmin's Marikana mine last year, the bloodiest security incident since the end of apartheid.

There has also been a public outcry over a $21 million state-funded security upgrade to Zuma's private home.