President Thabo Mbeki this week publicly acknowledged that he authorised Jacob Zuma's controversial letter to a top MP, a January 2001 communication that was key to the state's corruption case against his former deputy.

While Mbeki is ambivalent about whether he would be prepared to become a defence witness, he believes he can co-operate without having to stand in a witness box.

This comes as Zuma is preparing to pull out all the stops to prove his innocence.

In a wide-ranging interview with Independent Newspapers, Mbeki was asked whether he would be prepared to be a defence witness in Zuma's corruption trial. "It depends. One of the things we have to guard against, is acting in any way that reduces the esteem, prestige, authority of this institution (the office of the president)."

Mbeki said the information required for Zuma's defence was "simple and straightforward" and could be given without having to be called as a witness.

Zuma claims that although he signed the letter as leader of government business, it was actually written by Mbeki, working with a cluster of ministers, handling the fallout over the controversial arms deal.

He indicated last year that he might call Mbeki and other senior cabinet ministers as witnesses. Mbeki would be the first South African president to be a witness in a criminal trial if the defence went ahead.

The letter to Gavin Woods, the then chairperson of parliament's watchdog committee on public accounts, was used by the prosecution as evidence to convict Schabir Shaik and is expected to be equally crucial in Zuma's trial.

In his judgment last year, Judge Hilary Squires described the letter as "hostile", crowing and additional proof that Zuma had been carrying out the task for which he had allegedly been bribed by Shaik and French arms firm Thales - to protect them from further scrutiny in the arms deal probe.

Mbeki acknowledged he was party to the letter and the contents itself and made clear it was written with his authorisation. "It is actually in the communication, there is nothing hidden."

With acknowledgement to the Sunday Argus, Angela Quintal, Moshoeshoe Monare and Jovial Rantao.