National Director of Public Prosecutions Moketedi Mpshe could have made no other decision than to drop charges against ANC president Jacob Zuma, former judge Willem Heath said on Thursday.

"If he proceeded [with the prosecution] the judge would have ruled that it was an illegality and it would have come to an end," Heath said.

Heath was chairing an academic debate in Pretoria on the withdrawal of corruption charges against Zuma on 6 April.

The decision, Mpshe said at the time, was because there had been an abuse of process and manipulation by then-head of the Scorpions Leonard McCarthy and former head of the National Prosecuting Authority, Bulelani Ngcuka.

"Mpshe said there was conniving behind the scenes, to avoid Zuma becoming the president of the country and therefore they [Ngcuka and McCarthy] decided when to issue an indictment, an indictment to cast a shadow of a doubt," said Heath.

The timing around when to indict Zuma was provided to the NPA in the form of transcripts of telephone conversations between the then-prosecuting bosses.

"Mpshe disclosed that there was ulterior purpose [behind the decision to indict Zuma].

"If it?s done with ulterior purpose, it?s an illegality, not a technicality."

Heath said it was frightening that legal practitioners had been so quiet about the outcry over the NPA?s decision.

"The silence and the apathy of practitioners in South Africa after the announcement by advocate Mpshe is deafening."

He said it was common practice in South Africa and abroad for prosecutors to change their minds about continuing the process even after the case was already underway in court.

"No court of law can say to a prosecutor he made the wrong decision to withdraw [charges]. If you disagree, it?s no reason to criticise," said Heath.