DA leader Helen Zille, left, on Monday vowed to take her fight against the Erasmus Commission to the constitutional court.
She told the Cape Town Press Club the commission was an "illegal and unconstitutional hit squad" set up by the ANC to wrest control of Cape Town from the DA.
She took a swipe at commission chairman Judge Nathan Erasmus, saying he had allowed himself to be abused by the ANC.
His decision to chair it had undermined the independence of the judiciary required by the constitution, Zille said.
She said that the DA had "never, at any time" given anybody instructions to conduct illegal surveillance.
The commission was set up by Western Cape Premier Ebrahim Rasool in December 2007 to investigate the City of Cape Town's probe into controversial councillor Badih Chaaban.
The probe was meant to determine whether allegations that he had bribed other councillors to cross the floor were correct.
The commission's sittings were suspended last week pending the city's application to the Cape high court challenging the commission's legality. - Sapa