A powerful South Africa mining union on Monday backed Deputy President Cyril Ramaphosa to succeed President Jacob Zuma as the next leader of the ruling African National Congress (ANC).

Zuma is expected to stand down as ANC president at a party conference late next year, ahead of national elections in 2019 when his tenure as the country's leader will come to an end.

Ramaphosa, a wealthy businessman and founding member of National Union of Mineworkers (NUM), is likely to face strong competition if he does compete for the ANC leadership, including from Zuma's ex-wife, Nkosazana Dlamini-Zuma, who is currently head of the African Union.

Neither Ramaphosa or Dlamini-Zuma have said they will run.

A decision has been taken "to support the candidacy of Cyril Ramaphosa for president", NUM, one of South Africa's biggest unions, said in a statement.

Ramaphosa would be the first choice for many investors because he is more likely to support pro-business policies than many in the traditionally left-leaning ANC.