He has been called to give evidence about his role in the development of chemical and biological weapons during the 1980s.

Dr Basson is accused of masterminding a programme to develop chemical and biological weapons to be used against black people and anti-apartheid protesters.

He denies the allegation.

In a written statement, he said that South Africa's chemical and biological weapons programme was developed in response to the alleged use of chemical weapons by Cuban troops supporting the Angolan government against South African soldiers.

Bacteria against blacks

But Dr Basson's colleagues testifying to the Truth and Reconciliation Commission have said he was leading research into a vaccine to sterilise blacks and a bacterium that would kill only black people.

The former head of a military research laboratory, Daan Goosen, testified on Tuesday that Dr Basson had also discussed the possibility of killing the then imprisoned Nelson Mandela, by using carcinogens and arranging the supply of snake venom "to eliminate an enemy of the state".

Dr Basson's colleagues said he also oversaw the development at Roodeplaat laboratory, near Pretoria, of biological and chemical agents for use against anti-apartheid activists.

A web of front companies

According to testimonies to the Truth and Reconciliation Commission, Dr Basson alone controlled the network of front companies that were involved in the project.

The Truth and Reconciliation Commission established a list of 102 companies that are believed to have been linked to Dr Basson's 7th Medical Battalion.

The web of front companies used to cover up apartheid's biological and chemical weapons development is still under investigation.

Dr Basson had attempted to escape testifying to the Commission on the grounds that the hearing could prejudice a forthcoming criminal trial in connection with his work.

But the South African High Court judged in favour of the Truth Commission.

BBC's Greg Barrow:"a source of great concern to the Truth and Reconciliation Commission"

His appearance before the commission was delayed on Wednesday because he said his legal team had not been properly briefed.

A BBC correspondent in Johannesburg says that because the commission loses its power to subpoena people at the end of this week, Dr Basson may never have to testify if he can delay his hearing beyond Friday.