"It is neither possible nor desireable for the NPA to continue with the prosecution of Mr Zuma," Mokotedi Mpshe, head of the National Prosecuting Authority, told a press conference.

Zuma's lawyers had provided prosecutors with recordings that showed former NPA boss Bulelani Ngcuka and the ex-boss of the elite Scorpions investigative unit Leonard McCarthy conspiring over how to exact political gains from the case, said Mpshe.

The decision to drop the charges had not been based on the quality of the prosecution's case against Zuma, but because McCarthy's conduct had tainted the legal process, he added.

"Mr McCarthy used the legal process for a purpose outside and extraneous to the prosecution itself," he said. "It was pure abuse of process."

"It is not so much the prosecution itself that is tainted, but the legal process that is tainted."

McCarthy held repeated phone conversations with Ngcuka about the timing of the process against Zuma, coordinating it around key meetings of the ruling African National Congress (ANC), Mpshe said.

The announcement did little to clarify the merits of the case against Zuma.

It centres on accusations that he accepted bribes for protecting French arms company Thales in an investigation into a controversial multi-million-dollar weapons deal.

Since the investigation began in 2001, the charges have been repeatedly dropped and revived, sending shock waves through the country's political scene.

A 2008 court ruling -- since overturned -- implied that then-president Thabo Mbeki had interfered in Zuma's prosecution.

The ANC used the ruling to force Mbeki from office, leading his loyalists to launch a new breakaway party called the Congress of the People, which is challenging for power in the April 22.

As the election approaches, the furore over the corruption scandal has grown.

One month ago Schabir Shaik, a wealthy businessman convicted of bribing Zuma, was released on medical parole less than three years into a 15-year sentence.

His release raised eyebrows because just days earlier Zuma had told a local newspaper that he might use medical grounds as a reason to parole Shaik, a close aide and financial adviser, if he became president.

Faced with the challenge of proving that he did not receive the bribes Shaik was convicted of having given him, Zuma's lawyers brought prosecutors wire-tap recordings of McCarthy speaking about the case.

Despite the scandal clouding Zuma's campaign, a poll released on Friday found the ANC was still expected to secure more than 64 percent of the vote.