South Africa's top prosecutor, who is pursuing corruption cases against senior officials including ruling party leader Jacob Zuma, has been sent a letter laced with poison, his spokesman said yesterday.

The spokesman said Mokotedi Mpshe did not touch the letter himself, but National Prosecuting Authority staff who came in contact with the substance in the letter after it was opened were given medical treatment after developing rashes.

Spokesman Tlali Tlali said an internal investigation was yet to indicate who might have sent the letter to the acting head of the prosecuting authority, which has been at the centre of a power struggle between Mr Zuma and President Thabo Mbeki.

Mr Tlali said the letter did not appear linked to high-profile cases being handled by the prosecutor.

"On the face of it, the contents of the letter seem to suggest that it is from a whistleblower, as it alleges negative conduct by an NPA employee," he said in a statement.

Mr Tlali said the substance did not appear to be extremely dangerous but tests were still under way.

Police made no immediate comment on the letter.