Centre for Constitutional Rights director Johan Kruger said the ruling was a victory for the South African Constitution.

"The court's judgment reaffirmed our constitutional values, rights and principles, the superiority of the Constitution and the importance of abiding by the provisions of the Constitution," he said in a statement.

"This judgment by the Constitutional Court -- the ultimate guardian of the Constitution -- was a victory for the Constitution and a reaffirmation that no one, including the president, may act outside of the provisions of the Constitution."

The Constitutional Court made the ruling on Friday morning.

In reading the unanimous judgment, Judge Zak Yacoob said the court reached a number of legal conclusions, including that the NDPP's appointment was not a matter to be determined by the subjective opinion of the president.

"It was rather a jurisdictional prerequisite to be determined objectively," he said.

On May 8, the court heard an application by the Democratic Alliance to confirm a Supreme Court of Appeal ruling that the decision to employ Simelane was invalid.

President Jacob Zuma appointed Simelane as NDPP in November 2009.

Kruger said Zuma should explain his appointment because the Constitution required the government to be accountable, responsive and open.

The Council for the Advancement of the SA Constitution (Casac) welcomed the ruling and said it confirmed the constitutional principles that the office of NDPP had to be non-political and non-partisan.

"The Constitution requires the president to objectively ascertain whether the appointee is a fit and proper person with due regard to their conscientious and integrity..." it said.

Casac said that when Zuma appointed a new NDPP, he needed to consider whether the new appointee would be able to ensure that the credibility and neutrality of the National Prosecuting Authority was restored.

"The president has a legal duty to consider whether his chosen appointee will be able to ensure the NPA take decisions to institute criminal prosecution 'honestly, fairly and without fear, favour or prejudice'," it said.

The Inkatha Freedom Party said the fact that Zuma and his legal advisers had been prepared to go to court and had lost was proof that the African National Congress was a "failed government".

"This is yet another example of how government's incompetence is costing the taxpayers huge amounts in wasted legal costs," chief whip Koos van der Merwe said in a statement.

"The fact that the Constitutional Court gave judgment against the president of the country, is proof that our judiciary judge matters without fear or favour and true only to the Constitution and to their consciences."

On Friday afternoon, the presidency said it was studying the judgment.

"The presidency respects and will abide by the ruling of the Constitutional Court, which is the highest court in our democracy," spokesman Mac Maharaj said.

The report of the Ginwala Commission of Inquiry into Simelane's predecessor Vusi Pikoli's fitness to hold office had raised concerns about Simelane's understanding of the relationship between the justice department, where he was director general, and the NPA.

Ginwala, in her 2008 report, found his testimony contradictory and without basis in fact or law.

Yacoob said Simelane was evaluated in terms of the Ginwala inquiry and his evidence.

"[We] conclude that the evidence was contradictory and, on its face, indicative of Mr Simelane's honesty. It raises serious questions about Mr Simelane's conscientiousness, integrity, and credibility."