In an unprecedented show of unity, opposition parties are to establish a public trust fund in a bid to legally overturn the decision by National Prosecuting Authority head Mokotedi Mpshe to withdraw corruption charges against ANC president Jacob Zuma.

They urged all South Africans not to vote for the ANC in next week's elections to prevent the ruling party from attaining a two-thirds majority.

COPE leader Mosiuoa Lekota said on Wednesday that the IFP, ACDP, DA and the FF+ had joined hands with the Unashamedly Ethical organisation to "protect constitutional democracy".

This follows the DA's announcement on Wednesday that its lawyers had been instructed to investigate taking further legal action against Mpshe in the wake of an admission that he "cribbed" from an overturned Hong Kong judgment to explain his decision to drop the charges that haunted Zuma for eight years.

Addressing the media in Pretoria on Wednesday, opposition parties said they were concerned that the ANC's two-thirds majority was a threat to the rule of law.

"We believe that there is a need to urgently address the issues of the legal validity of the grounds given for the decision to discontinue the prosecution of Jacob Zuma, the legality of the tapping of telephones and the apparent abuse of power by the state institutions in order to satisfy political ends," said Lekota.

They were also concerned about Zuma's statements which called into question the powers and the integrity of the Constitutional Court and the incumbent judges.

"We believe that in order to prevent the future abuse of state institutions and to protect the rule of law it is imperative that no single political party has the necessary majority to change portions of our constitution which seek to entrench the Bill of Rights and the power and jurisdiction of independent institutions as established under our national constitution.

"With these concerns in mind the parties have resolved to join Unashamedly Ethical to establish a fund to receive donations to explore every legal avenue to address the points highlighted," said Lekota.

They called on all other opposition parties to join them to ensure that no party would get a two-thirds majority.

They would use the fund, to be established this week, to protect state institutions and chapter 9 institutions such as the South African Human Rights Commission and the Public Protector.

The ombudsman for Unashamedly Ethical, advocate Keith Matthee, said his organisation - a non-partisan body which promotes good values and clean living among all South Africans - was concerned about Zuma's statements on the judiciary.