The investigation against African National Congress president Jacob Zuma is based solely on evidence, not a political conspiracy, the National Prosecuting Authority said on Tuesday.

"The allegations of political conspiracy on the part of the NPA are still not supported by any credible evidence, but rest rather on speculation and innuendo," a statement from NPA spokesperson Tlali Tlali read.

"They are not strengthened but remain mere repetition and unsubstantiated."

On Monday Zuma's attorney filed papers with the Pietermaritzburg High Court arguing that his forthcoming corruption trial should be declared invalid.

In a preamble to the history of his case, Zuma said: "In dealing with these events I shall endeavour to steer away in so far feasible from contentious issues such as the political motives and stratagems which I verily believe were and are the driving forces behind many of the actions of the prosecuting authorities and those who influence these."

He also added a note before a summarised personal history which included imprisonment on Robben Island, being deputy president of the country, an MEC in KwaZulu-Natal and head of Intelligence of the ANC.

"I respectfully, but specifically, make this point because I verily believe that I have in the past on occasion been subjected to such "special" treatment to my detriment."

The NPA said it had taken note of this and all his allegations would be "fully traversed in the papers" when its answering papers were filed "in due course".

"It is apparent that Mr Zuma has seen fit to repeat yet again his allegations that his prosecution is political. This is not correct... "

The latest application was expected to be heard in the Pietermaritzburg high court on 4 and 5 August and would come before the start of his corruption trial on 14 August.

The NPA first announced plans to prosecute him in June 2005, and then again in December 2007 after the first attempt stalled to give the NPA more time to prepare and to wait for the outcome of various court challenges to aspects of the investigation.

In addition to corruption charges, Zuma also faces fraud, racketeering and money laundering charges. Co-accused, arms company Thint, faces charges of racketeering, money laundering and corruption.

At a conference on Monday, Zuma said he was "not even half guilty" of the charges.

Sapa