The controversy around Willem Heath, the newly appointed head of the Special Investigating Unit, continued this weekend.

In an open letter, published in various Sunday publications, former national director of public prosecutions Bulelani Ngcuka told Heath to stop "sulking".

He dared Heath - who made a series of damning allegations against Ngcuka and former president Thabo Mbeki in an interview with City Press last weekend - to produce evidence of wrongdoing.

"The allegations that you make against me personally, and the office of the national director of public prosecutions during my tenure, are very serious. They include the scurrilous charge that I allowed myself and the National Prosecuting Authority to be dictated to by the former president [Mbeki] in the exercise of my authority, to institute corruption and rape charges against President Jacob Zuma," Ngcuka wrote.

Referring to Heath's departure from the SIU, Ngcuka continues: "One, however, understands that a man so bitter at being 'excluded' from office and rendered a pauper as a result of such 'exclusion', would be blind to the facts and be inclined to lash out at all those whom he misguidedly perceives to have conspired to his prejudice."

Ngcuka's letter was written amid growing concerns about Heath's appointment to the SIU by Zuma less than two weeks ago.

On Wednesday the president, in a radio interview, said he had asked Justice Minister Jeff Radebe for legal advice on Heath's comments.

On Saturday Radebe confirmed that he was investigating the remarks made to City Press and hoped to report to Zuma soon.

Heath has also come under fire from Mbeki, whose office said on Thursday that the former president demanded proof of the former judge's allegations.

"The legal representative of the Thabo Mbeki Foundation has today formally approached the government to provide evidence of all the allegations made by advocate Heath."

In an interview with the Sunday Times in 2005 Heath was asked whether Mbeki was justified in sacking Zuma. Heath replied: "Yes, I think so, most definitely. In addition to that, I would say that, in any democratic society, Zuma would have resigned."

Zuma's spin doctor, Mac Maharaj, has been quick to create distance between the president and Heath. On Saturday he confirmed that Zuma had received a letter from Mbeki's lawyers, and said Heath had spoken in his personal capacity.