South Africa's ruling ANC leader Jacob Zuma's former financial adviser has got parole on medical grounds after 28 months in jail for fraud in a case that also led to charges against Zuma, officials said on Tuesday.

The decision to parole Schabir Shaik, sentenced to 15 years in 2005, drew accusations of preferential treatment from the opposition. Corruption is a central issue in the campaign for the April 22 general election, although Zuma is almost certain to become president despite the graft charges against him.

Shaik has spent most of his imprisonment in hospital and local media said he had suffered high blood pressure and depression.

"Mr. Shaik has been granted medical parole," said Manelisi Wolela, chief director of communications for the Department of Correctional Services.

The official opposition, the Democratic Alliance, said medical parole was to allow terminally ill prisoners to die a dignified death and called for disclosure of the reasons to free Shaik.

"This will do much to dispel the notion that double standards are being applied," the party said in a statement.

Shaik was found guilty of trying to solicit a 500,000 rand (now $48,000) a year bribe for Zuma from French arms company Thint in return for protecting it from an investigation into an arms deal in the late 1990s. Another count said Shaik paid Zuma to use political influence.

Zuma told The Weekender newspaper that he would pardon Shaik if he became president, as he is expected to as leader of the dominant African National Congress.

Shaik's conviction led to former President Thabo Mbeki firing Zuma as deputy president in June 2005, spawning divisions in the ANC that led to Mbeki's ousting last year and a splinter group forming a new party.

Zuma has approached the highest court, the Constitutional Court, to try to get the charges against him dropped completely. If he fails, he could find himself in the dock as president.

Zuma has denied the corruption charges which he says are politically motivated.

Shaik's release will be a hot campaign topic for opposition parties because of his connection with Zuma but is unlikely to influence voters, an analyst said.

"Political parties are going to argue that (Shaik) got preferential treatment," said Aubrey Matshiqi, political analyst at the Centre for Policy Studies.

"But, even if (voters) care about this release of Shaik it is not going to have a substantial impact on electoral patterns."