South African President Jacob Zuma looks on before delivering an address in Polokwane, South Africa, December 20, 2007.[Photo/Agencies]

PRETORIA - South Africa's High Court ruled on Friday that a 2009 decision to drop 783 corruption charges against President Jacob Zuma was irrational and should be reviewed.

The decision in April 2009 to set aside the charges allowed Zuma to run for president in elections the same month.

National prosecutor Mokotedi Mpshe's decision at the time was based on phone intercepts presented by Zuma's legal team that suggested the timing of the charges against Zuma in late 2007 may have been part of a political plot against him.

However, Judge Aubrey Ledwaba said Mpshe's thinking and behaviour was irrational, especially his failure to disclose his decision to prosecutors until the moment he announced it to the nation at a news conference.

"If the decision had been rational and above board, why the secrecy?" Ledwaba said.

The ruling adds pressure on Zuma, who has faced calls for his resignation even from within inside the ruling African National Congress since a damning constitutional court judgment against him last month.

South Africa's rand rose towards a four-month high after the ruling, while government bonds also firmed.

"Mr. Zuma should face the charges as applied in the indictment," Ledwaba said, summarising the unanimous ruling.

It was not immediately clear whether Zuma would appeal.

The judgment does not automatically reinstate the charges against Zuma, a decision that can only come from the prosecuting authorities.

Shaun Abrahams, head of the National Prosecuting Authority, told Reuters he was studying the ruling.

Opposition Democratic Alliance leader Mmusi Maimane, whose party brought the court application, demanded the charges against Zuma be reinstated.

"Jacob Zuma is not fit to be the president of this country," he told reporters.

"The decision that they took was irrational and we still maintain that Jacob Zuma is corrupt. Jacob Zuma must face the full might of the law. He has already violated the constitution. There is no debate about that."

The hundreds of corruption charges relate to a major government arms deal arranged in the late 1990s. Zuma said last week that an investigation into the deal had found no evidence of corruption or fraud.