Former South African president Jacob Zuma has been expelled from the African National Congress after backing a rival party in May's parliamentary election, the ANC said on Monday.

Zuma was suspended from his long-time party in January after announcing he would support the new uMkhonto we Sizwe (MK) in the general election. He later became the leader and face of MK.

MK did better than expected in the poll and won about 15% of the vote, a major factor in the ANC's loss of its majority for the first time since the apartheid era.

"Former President Jacob Zuma has actively impugned the integrity of the ANC and campaigned to dislodge the ANC from power, while claiming that he had not severed his membership," ANC Secretary General Fikile Mbalula told the media.

"This conduct is irreconcilable with the spirit of organisational discipline and letter of the ANC Constitution."

The ANC won 40.18% of the vote in the election, down from 57.50% it gathered in 2019, forcing it into a coalition government for the first time since it took power at the end of white minority rule in 1994. MK is now the official opposition.

MK responded to media reports that Zuma had been expelled in a statement earlier on Monday, criticizing the ANC's process and condemning "grave injustices" against Zuma.

"President Zuma will engage his legal team to urgently determine the course of action," it said.

Zuma will have 21 days to appeal the decision, the ANC said.

Zuma, whose nine years in office from 2009 were marked by corruption scandals and sluggish economic growth, has been at odds with the ANC's leadership since he was forced to quit as party leader in 2018. He has repeatedly lashed out against his successor, President Cyril Ramaphosa.