Justice Minister Penuell Maduna is expected today to announce the retired judge who will head a judicial commission of inquiry into allegations that Scorpions boss Bulelani Ngcuka was an apartheid spy.

Maduna will also set out the commission's terms of reference, chief government spokesman Joel Netshitenzhe said at a cabinet briefing yesterday.

The commission was appointed by President Thabo Mbeki.

Deputy President Jacob Zuma and former transport minister Mac Maharaj - both of whom were senior members of the intelligence structures of the ANC and both of whom are under investigation by the Scorpions - have charged that Ngcuka was investigated for being an apartheid spy.

The government is also investigating the "serious matter" of leaks from a cabinet meeting a month ago at which Zuma reportedly launched a scathing attack on Maduna, who takes political responsibility for Ngcuka's National Prosecuting Authority and the Scorpions investigating unit.

And in parliament yesterday, Mbeki vigorously defended Zuma against calls for disciplinary action to be taken against him "simply on the basis of allegations".

Mbeki made no reference to the appointment of the judicial commission during President's question time.

The commission was appointed, Netshitenzhe said, "to avoid witch-hunts" in the media.

He said it was important to take into account "the intricate struggle to forge a new nation".

"... the purpose here is to ensure that this matter is urgently processed and ensure that in a matter of weeks findings can be made so that we do not have a situation in which the unit finds itself debilitated by these kinds of (spy) allegations," Netshitenzhe said.

"As government we want to urge anyone who claims to have information on this particular matter relating to Mr Bulelani Ngcuka to take it to the commission rather than airing it through the media.

"Our own experience as well as that of other countries has shown that the interest of the country will not be served by witch-hunts. If we allow this matter to be handled in a selective and disorderly manner you can find yourself on a slippery slope of mutual recrimination and subversion of human rights," he said.

"Therefore we urge that these matters ... should be handled by the commission instead of through the media."

Regarding the investigation into the leak from cabinet, Netshitenzhe said such a leak was "a serious matter" as sensitive matters such as the country's security, the markets and international relations were routinely discussed..

In his question session yesterday, Mbeki expressed confidence in Ngcuka.

Answering a question from Democratic Alliance (DA) leader Tony Leon, Mbeki poured scorn on the DA suggestion that Zuma should be "sacked" until the corruption allegations against him are refuted.

Leon is in London, but DA chairman Joe Seramane engaged the president in follow-up questions.

Mbeki trod a fine line in the acrimonious stand-off between Zuma and Ngcuka by insisting that Zuma was entitled to the presumption of innocence until proven guilty while at the same time expressing his confidence in Ngcuka, Zuma's adversary.

Mbeki said while the allegations against Zuma were just that - allegations - he was "confident" that Ngcuka would discharge his responsibilities as required in law.

"I have not and will not question his decisions," Mbeki said.

The president said that nobody could produce evidence about corruption in the arms acquisition process, but stopped short of saying he had the fullest confidence in his deputy.

Mbeki said the DA should substantiate its allegations to justify their suggestion that he should remove Zuma.

He would not take disciplinary action or any other action "simply on the basis of allegations, whoever makes these allegations".

Louis Green, vice-president of the African Christian Democratic Party, in his question to Mbeki noted that Ngcuka had decided not to prosecute Zuma even though the NDPP had claimed that he had prima facie evidence that Zuma was involved in arms deal corruption.

In terms of internal cabinet rules, which state that a member may not expose himself to conflict of interest, Mbeki may discipline Zuma on the basis of persuasive prima facie evidence, Green said.

He asked Mbeki whether he would use these measures to sanction Zuma.

With acknowledgements to Jeremy Michaels, Andre Koopman and the Cape Times.