South Africa's main opposition party won the right on Tuesday to review potentially damaging documents from a discontinued corruption case against President Jacob Zuma in 2009, a move that could hurt Zuma's chances of re-election as ANC leader.

The president, who seeks re-election as African National Congress (ANC) leader late this year, had been expected to win easily, virtually securing him another five-year term in office as president because of the party's dominance over politics.

Tuesday's court ruling allows the main opposition Democratic Alliance (DA) access to papers from a case against Zuma over a tainted arms deal, which was dropped by prosecutors just weeks before a presidential election in 2009, clearing the way for him to win the race.

The Supreme Court of Appeal said in its ruling on Tuesday that it was in the public interest to allow the DA access to the papers from the case.

The South African Presidency said in a statement it was studying the ruling, noting the decision not to prosecute stands.

It is not known what is contained in the documents or how much of the documents the opposition would be allowed to see. The DA has been eating into the ANC's dominance but trailed the party by about 40 percentage points in the last nationwide election in 2011.

Zuma could be damaged if the documents contain evidence of criminality, especially since he has said stamping out corruption is one of his top priorities.

ANC challengers could use graft allegations to damage Zuma, who currently commands enough support in the party to fend off leadership rivals.

The ANC frowns on open campaigning for party posts well ahead of its election but likely challengers include Deputy President Kgalema Motlanthe and housing minister Tokyo Sexwale.

In the court case, Zuma was implicated in a decade-old arms deal that led to several ANC heavyweights being convicted of bribery. He had been charged with corruption, fraud, money laundering and racketeering.

Zuma has said he was the victim of a political conspiracy while his opponents outside the party have accused the ANC of back-room deals to clear his name.

The National Prosecuting Authority said in 2009 there were abuses in the police investigation and it was dropping the charges without revealing details of their investigation.

(Additional reporting by Cosmas Butunyi; Editing by David Dolan and Susan Fenton)