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 for rashes.

Spokesman Tlali Tlali gave no indication who he thought might have sent the letter to the acting head of the prosecuting authority, which has been at the centre of a power struggle between Zuma and President Thabo Mbeki.

"The letter was received on Thursday. It was intended for the national director," said Tlali, adding that some of the staff developed a rash after being exposed to the contents of the letter.

Tlali said an internal investigation had so far not pointed to who could have intended to harm the NPA director. He said the substance did not appear to be extremely dangerous but tests were still underway.

Police made no immediate comment on the letter.

Mpshe became acting head of the prosecutors' office after Mbeki suspended his predecessor. That prompted critics to accuse Mbeki of using the judiciary and state institutions against opponents such as Zuma.

After defeating Mbeki in the contest to lead the ruling African National Congress last year, Zuma is well placed to become president in 2009 but first needs to beat corruption charges which he says are politically motivated.

The prosecutor has denied that Mbeki was behind the charges against Zuma. He also has a corruption case against police chief Jackie Selebi, an ally of Mbeki.

Zuma is due to appear before the Constitutional Court on Tuesday to try to stop prosecutors using evidence seized from him during raids in 2005 against him in the corruption trial, which is set to start in August.

During the apartheid era, police sometimes used letters or items of clothing laced with poison to target opponents. In 1989, they tried to kill ANC activist Frank Chikane with poisoned underwear. He is now a top adviser to Mbeki.