"With the matter regarding the resignation of a senior manager, the SIU would like to state that the member mentioned in the DA statement, Mr Sello Makhubela is not a senior manager but an IT specialist," said SIU acting communications manager Boy Ndala.

"The SIU confirms that Mr Makhubela has resigned, however the reasons cited in the DA statement as being political is untrue," he said in a statement.

The Democratic Alliance said on Thursday it had learnt that "another senior manager, Mr Sello Makubela has resigned from the SIU, allegedly citing that he can no longer deal with the politics".

DA MP Debbie Schafer said he was the sixth manager to resign since Novula Mokhatla was appointed as acting head of the anti-corruption unit after Willie Hofmeyr, the deputy director of public prosecutions, was removed from the position late in 2011.

"We have also ascertained that at least one complaint was lodged with the Public Protector last year by a member of the SIU executive committee. It is clear that an investigation into the unit is desperately needed," Schafer said in a statement.

However, Ndala said: "Regarding the resignation of six senior managers and demotion of others, the DA statement is baseless and lacks factual information. The DA's information and sources are questionable."

He said an executive committee member had lodged a complaint with the Public Protector in July 2011, which was before advocate Nomvula Mokhatla was appointed as acting head of the SIU.