South Africa’s highest court on Monday ruled that former President Jacob Zuma was not eligible to serve in Parliament, a decision that may deepen political turmoil in the country just over a week before a crucial national election.

The decision could draw the curtain on the political career of the 82-year-old Mr. Zuma, a former anti-apartheid hero who once led the liberation party, the African National Congress. Mr. Zuma had a bitter falling out with the A.N.C. last year after announcing he was supporting a new political formation.

The Constitutional Court, overturning a special electoral court’s earlier decision, ruled that Mr. Zuma could not stand as a candidate in the May 29 election because of a past criminal conviction.

Even though he cannot serve in Parliament, Mr. Zuma’s face will still appear on the ballot next to his new party, uMkhonto weSizwe, because he is registered as its leader, according to the Electoral Commission of South Africa.

As a populist figure who attracts a devoted following, Mr. Zuma’s image could be enough to lift his party’s fortunes and hurt the A.N.C., which is fighting to maintain the absolute majority it has held since the start of South Africa’s democracy 30 years ago.

Mr. Zuma resigned from the presidency in 2018 amid widespread protests over allegations of sweeping corruption within his government. Three years later, he was convicted and sentenced for failing to testify at a public inquiry on corruption.