South Africa’s former president Jacob Zuma has been expelled by the African National Congress after he backed a rival political party that helped cost the ANC its parliamentary majority in recent national elections.

“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,” the party said in a statement on Monday.

The ANC – which had governed South Africa alone since the country’s first fully democratic elections in 1994 – slumped from 57.5% of the vote in 2019 to 40.2% in May as it was deserted by voters.

Poor public services and chronically high unemployment played a part in the collapse, but a significant portion of its base flocked to Zuma’s new party, uMkhonto weSizwe (MK). Named after the ANC’s armed wing during apartheid, it became the country’s third largest party, with 14.6% of the vote.

Zuma, 82, was president from 2009 to 2018, but was forced to resign amid corruption allegations. The launch of the MK party in December was seen by analysts as an attempt to get revenge on Cyril Ramaphosa, his successor as ANC leader and as president of South Africa.

The MK party accused the ANC of running a “kangaroo court”. It said Zuma would consult his legal team to decide his next steps.

“We don’t know why you [set up a new] party and then you argue for your membership, when you have taken a conscious decision to leave the ANC,” the ANC secretary general, Fikile Mbalula, told a press conference. “You have brought us below 50%. We are grappling with that.”

The MK party said in its election manifesto that it wanted to get a two-thirds majority so it could change the constitution to make parliament the supreme power in the country. It also pledged to nationalise banks and expropriate land without compensation.

A judicial inquiry alleged that Zuma replaced competent officials with loyalists and influenced the awarding of large contracts, to benefit the Indian businessmen brothers Atul, Ajay and Rajesh Gupta, a scandal known as state capture. He is also due to go on trial next year on charges that he accepted bribes in a 1999 arms deal. Zuma has always denied corruption allegations.

Zuma has 21 days to appeal against his expulsion, the ANC said.