The National Intelligence Agency's (NIA) deputy head, Arthur Fraser, leaked secret recordings to ANC leader Jacob Zuma's lawyers ? evidence that ultimately halted his prosecution, according to a report.

Quoting from three independent sources, the weekly Mail&Guardian said Fraser was once closely linked to former president Thabo Mbeki but had "changed his political tune some time ago".

"The office of the Inspector General (of Intelligence) is investigating it," NIA spokeswoman Lorna Daniels told Sapa.

"We strongly deny it. We have no further comment," she added.

Based on information obtained from the recordings, the National Prosecuting Authority (NPA) this week announced it would halt the prosecution of Zuma, who was facing fraud and corruption charges.

The charges were dropped by the High Court in Durban on Tuesday.

The tapes are recordings of phone conversations between former NPA head Bulelani Ngcuka and former Scorpions boss Leonard McCarthy, discussing, among other things, the timing of an announcement to re-charge Zuma.

These conversations happened in the weeks before the ANC's conference in Polokwane in December 2007, when Zuma was elected its new leader, in a race closely contested by Mbeki.

NPA Acting National Director Mokotedi Mpshe said on Monday the recordings showed that McCarthy had abused the legal process.

According to the Mail&Guardian, Fraser, who joined the NIA in 1995, could not be reached for comment. His secretary told the newspaper he was "not in Gauteng, but in another province and can't receive any calls".

An unnamed senior legal source told the paper: "We understand Fraser felt the need to ingratiate himself with the new administration of Zuma and handed the NIA tapes over."

The chief of operations at the Inspector General of Intelligence's office, Imtiaz Fazel, confirmed it was investigating "circumstances surrounding the interception of the voice communications of certain individuals".