Minister Fikile Mbalula calls ANC MPs who vote against the party line "suicide bombers" who will be punished. (Picture: Phando Jikelo)

South African President Jacob Zuma, who has been weakened by growing criticism from within the ruling ANC party, will face a vote of no confidence in parliament on August 8.

The new date is five days later than originally planned because of a scheduling clash with a cabinet meeting, parliamentary Speaker Baleka Mbete said on Sunday.

Mbete, a Zuma ally, will decide whether the vote is secret after a case brought to the Constitutional Court by opposition parties, who have called for ANC lawmakers to "vote with their conscience".

Opposition parties have pushed for the vote to be held in secret, hoping to encourage ANC lawmakers to vote Zuma out of office after a series of corruption scandals.

But the president retains strong support from many lawmakers in the African National Congress.

In the last two years, Zuma has easily survived three votes of no confidence and a separate parliament vote to remove him from office.

The latest no-confidence vote was initially scheduled for April but delayed to allow the court to rule. The ANC has vowed to defeat the motion.

Mbete has yet to decide whether the latest ballot will be held in secret after the country's top court ruled in June that secret votes may be held for motions of no confidence.

Zuma is due to step down as ANC head in December and as national president ahead of the 2019 election.

On Friday, he opened a party conference admitting that the ANC was beset by corruption and divisions that could threaten a hold on power that has prevailed since the end of apartheid in 1994.

At the congress the party ramped up its support for Zuma, saying members of its parliamentary caucus who defied the party line were "suicide bombers" who would face punishment if they voted with the opposition.

Zuma has survived four no-confidence votes during his eight years in power owing to loyal voting by ANC lawmakers, who form a strong parliamentary majority, although cracks in unity have begun to appear with a number of allies threatening to break ranks.

Zuma's administration has been beset by scandals and criticised for failing to address serious economic problems, with calls for his resignation sharpening after he fired the finance minister in March, triggering a double downgrade of the country's debt to "junk".

On Sunday, however, police minister and head of the ANC's sub-committee on organisation, Fikile Mbalula, described party lawmakers thinking of voting Zuma out as "suicide bombers".

"Members of the ANC have no right to represent their own jackets in parliament. They represent the organisation. They are suicide bombers (if they vote against Zuma)," Mbalula told reporters at a media conference. "The ANC will apply disciplinary procedures for those want to vote using their so-called conscience," he said.