CAPE TOWN (Sapa) - The controversial Erasmus Commission was set up for political reasons and was unconstitutional, a full bench of the High Court ruled on Monday.

The judges also said that the appointment of a serving judge to chair the commission was incompatible with the separation of powers and was therefore unlawful.

And they found that both commission chairman judge Nathan Erasmus and Western Cape police commissioner Mzwandile Petros illegally passed on information to provincial premier Ebrahim Rasool.

The ruling, handed down by KwaZulu-Natal judges Kevin Swain and Chris Nicholson in the Cape High Court on Monday morning, followed a challenge by the City of Cape Town and the Democratic Alliance, which leads the city government.

It was hailed by city mayor and DA leader Helen Zille as a victory for democracy and the Constitution.

"The court has... confirmed our view that the Erasmus Commission was a blatant abuse of power by the ANC-led provincial government," she said.

"We hope that this will be the last attempt by the ANC to unseat the multi-party government of Cape Town until the next local government elections."

The commission was set up last year by Rasool ostensibly to probe the legality of the city's investigation of renegade councillor Badih Chaaban, who was suspected of floor-crossing bribery.

Rasool re-established the commission in March this year following an initial challenge by the city.

However judge Swain, reading out key sections of the 156-page judgment, said Rasool's move had to be seen against the background of the antagonism between the ANC and the DA in the Western Cape, and the struggle for control of Cape Town.

Rasool had not given the court any credible reason for establishing the commission, and had acted arbitrarily and unlawfully in doing so.

"His only motive on the evidence in establishing the second Erasmus Commission, must have been to embarrass or discredit political opponents, particularly the DA," Swain said.

"In this context I find the inference irresistible that one of the reasons why the premier appointed a judge to chair the commission, was in order to cloak his ulterior motive with the neutral colours of the judicial office."

He said Erasmus breached the regulations governing the commission by giving a so-called "interim report" report to Rasool when the body was about to be reconstituted.

"Consequently, the information contained in the summary of evidence was obtained by the premier unlawfully," he said.

Petros likewise acted illegally when he passed on to Rasool information that the police obtained in a search of the home of private investigator Philip du Toit, a key player in the Chaaban affair.

That information should have been fully investigated by the police and then handed to the director of public prosecutions, Swain said.

The city had argued that Petros had been seeking to further the interests of the ANC in the Western Cape, conduct which would be unconstitutional.

"I have a grave suspicion that Commissioner Petros may have had such an objective in mind in furnishing the information to the Premier [but] I cannot on these papers, find as a fact that this was his objective," he said.

On Erasmus' appointment, Swain said the terms of reference of the commission had effectively given the judge prosecutorial powers, which was a gross violation of the principle of separation of powers, and was therefore unlawful and invalid.

Before accepting an appointment to chair a commission, a judge should be satisfied that it was not incompatible with his or her judicial office.

"Simply put, the involvement of Erasmus J in the commission has unnecessarily involved the judge in the political controversy surrounding the commission, which may damage the confidence of the public in the judiciary's core function of determining matters in court," Swain said.

"It seems to me that at this early stage of our fledgling democracy, and with the vital object of preserving public confidence in the independence of the judiciary, active judges should as a matter of principle, not chair commissions of inquiry."