"... the DA will demand to know its reasons for doing so (for dropping the charges).

"Should the reasons not be satisfactory and accompanied by proper disclosure, we will institute legal proceedings to ensure that justice is served," said Democratic Alliance leader Helen Zille in a statement on Thursday.

The NPA was deciding whether to drop 16 criminal charges ? one of racketeering, one of money-laundering, two of corruption and 12 of fraud, against Zuma, based on representations his legal team made to it.

"Selected leaks to the press indicate that the NPA is hopelessly compromised because of political interference.

"It appears that the NPA might drop the charges against Jacob Zuma because it fears that Zuma will use his own corruption trial to demonstrate that the NPA was complicit in a political conspiracy against him," Zille said.

Irrespective of the role the NPA played in the alleged conspiracy, she said, "... the fact remains that Jacob Zuma has a case to answer".

'Strong grounds to proceed'

Quoting Supreme Court of Appeal Judge Louis Harms, the opposition party leader said there were "strong grounds" to proceed with Zuma's prosecution for the 783 bribes he was allegedly involved in with his former financial advisor and convicted fraudster Schabir Shaik.

"As Judge Louis Harms of the Supreme Court of Appeal noted, in overturning the Pietermaritzburg High Court's 2008 verdict on Zuma's prosecution: 'A prosecution is not wrongful merely because it is brought for an improper purpose. It will only be wrongful if, in addition, reasonable and probable grounds for prosecuting are absent'."

Proceeding with Zuma's prosecution was the NPA's "duty and responsibility", she said.

Earlier this week, Zuma said it was "hypocritical" for opposition parties to pre-empt the NPA's decision.

This was after the DA made submissions to the authority on why the charges against him should not be dropped.

"It is hypocritical to say you respect the Constitution and the rule of law and when the processes of the law are happening within the law... both the accused and the NPA are operating within the law, why do you jump and interfere?

"You even attack a decision that does not exist," Zuma said.

The prosecuting authority, after marathon talks on the way forward on Monday and Tuesday, said it would announce when it would make a decision on Zuma this Friday.

NPA spokesperson Tlali Tlali said there were certain "pieces of the puzzle" the authority needed to work through before reaching a final decision.

These "pieces" formed a "critical part" of the decision-making process, he said.

The NPA decision could end a lengthy legal battle with the ruling party president and clear the way for him to assume the presidency without having to appear in the docks during his time at the Union Buildings.