South Africa's top court ruled on Monday that former president Jacob Zuma was not eligible to run for parliament in this month's election, a closely-watched decision as it could affect the outcome.

Zuma, who was forced to quit as president in 2018, has fallen out with the governing African National Congress (ANC) and has been campaigning for a new party called uMkhonto we Sizwe (MK) named after the ANC's formed armed wing.

Opinion polls suggest the ANC's majority is at risk after 30 years in power, and MK represents a threat to it, especially in Zuma's home province of KwaZulu-Natal where he is popular.

The case before the constitutional court stems from a decision in March by South Africa's electoral commission to disqualify Zuma on the basis that the constitution prohibits anyone given a prison sentence of 12 months or longer from holding a parliamentary seat.

In 2021, Zuma was sentenced to 15 months in jail for failing to appear at a corruption inquiry.

In April, a court overturned the disqualification, saying the relevant section of the constitution applied only to people who had a chance to appeal against their sentences, which had not been Zuma's case.

The electoral commission then challenged that decision in the constitutional court.

"It is declared that Mr Zuma was convicted of an offence and sentenced to more than 12 months' imprisonment, ... and is accordingly not eligible to be a member of, and not qualified to stand for election to, the National Assembly," the constitutional court said on Monday in its ruling.

In 2021 Zuma's jailing triggered riots in KwaZulu-Natal in which more than 300 people died and which morphed into a wider spate of looting.