An encrypted letter and the disappearance of two witnesses are understood to be among the main reasons why the case against Deputy President Jacob Zuma has crumbled.

The main evidence against Zuma is built around encrypted correspondence between arms deal beneficiaries in the Thomsons group.

The correspondence described a meeting that took place at the Durban Holiday Inn in 1998 during which a bribe was solicited. The meeting was attended by Zuma and his financial adviser, businessman Schabir Shaik, along with Alain Thetart and Jean-Pierre Perrier.

In his response on Sunday, Zuma said he had only seen an English version of the letter and questioned the existence of a French version. In order for the case against him to go ahead, it is understood that Thetart and Perrier have to testify - but both are missing.

Bulelani Ngcuka, the National Director of Public Prosecutions (NDPP), said at the weekend: "We will be referring the evidence we have against Alain Thetart, Perrier and international companies like Thomson to the French authorities for them to take action."

Earlier, he said French authorities had yet to respond to and comply with "our request for mutual legal assistance sent to them earlier this year" for the two to be extradited.

It is understood that the Scorpions have little hope that Thetart and Perrier would appear and testify against Zuma - thereby implicating themselves in wrongdoing.

Meanwhile Zuma has accused Ngcuka of handing out the "equivalent to a judgment" against him and of leaving a cloud hanging over him.

On Sunday afternoon, Zuma finally responded to Ngcuka's announcement that he would not be prosecuted over the multibillion-rand arms deal, despite indications that there may be a corruption case for which the deputy president will have to answer. Ngcuka said the decision was taken in spite of a recommendation by the investigating team that Zuma be criminally charged.

Zuma also lashed out at Justice Minister Penuell Maduna's statement on Saturday and said it was a "sad moment that the investigators have concluded that I have a case to answer".

"What is this case? Where should I answer it?" Zuma asked in a statement.

"I intend to take this matter further. I shall consult with my lawyers and should be able to respond comprehensively to the NDPP's statement during the course of the week."

He said the "purpose of the latest public announcement is to leave a cloud hanging over my integrity after a long trial by media. The hallowed principle of presumption of innocence until proven guilty has been completely ignored.

"No matter what the NDPP says, there can only be one reason I am not being charged - there is no case against me."

On Saturday, Ngcuka said: "We have concluded that while there is a prima facie case of corruption against the deputy president, our prospects of success are not strong enough. That means we are not sure if we have a winnable case. Accordingly, we have decided not to prosecute the deputy president."

He said the decision could be reviewed if new evidence arose. However, Shaik would be charged with corruption, fraud, theft of company assets, tax evasion and reckless trading. He was due to appear in the Durban magistrate's court on Monday.

In other developments on Sunday, the opposition called on Zuma to resign, while the African National Congress declared that there would be no internal investigation of the allegations against Zuma.

Parliament is, however, waiting for evidence from Ngcuka to decide whether it would investigate alleged non-declarations of gifts and donations received by Zuma.

National Assembly Speaker Frene Ginwala said on Sunday that she had not received any request or evidence from Ngcuka. She promised that the issue would automatically be referred to the joint ethics committee.

With acknowledgements to Christelle Terreblanche and The Star.