South Africa's former President Jacob Zuma has requested the head of a commission investigating corruption during his presidency to step aside over alleged "bias", his lawyers said.
Mr Zuma has failed to re-appear before the commission as has been requested.
But Deputy Chief Justice Raymond Zondo has set a "non-negotiable" date for him to appear.
Allegations of corruption dogged Mr Zuma's presidency and led him to resign in February 2018.
The former president's lawyer has said that he would not take part until Mr Zondo is replaced.
In a letter to the commission, Eric Mabuza said: "We are instructed to seek your recusal as chairperson of the commission on the ground that our client reasonably apprehends that you have already adopted a biased disposition towards him."
He said that Mr Zondo was no longer capable of "exercising an independent and impartial mind".
"President Zuma has always expressed his willingness to cooperate with the commission. This is in spite of his reservations about the legality of the commission and in particular, your suitability as chairperson, given your personal relations with him".
The inquiry, known as the Zondo Commission, was established to investigate the "state capture" scandal during Mr Zuma's tenure as president.
This centred around allegations about an Indian business family, the Guptas, who won lucrative contracts with state companies.
The family has also been accused of trying to influence political decisions, including the naming of ministers.
The Guptas have said that there were no cases to answer and that they were in the process of clearing their name "in the face of unfounded media allegations".
Delay or due process?
Analysis, Pumza Fihlani, BBC News, Johannesburg
Even though he has repeatedly denied any wrongdoing, some say he has used his qualities as a tenacious fighter to stay out of the courts, baffling for someone who has always insisted that he would take up the first opportunity to clear his name.
While some see this latest wrangling over the Zondo Commission as an attempt to delay the investigation and buy time to plan, his supporters see this merely him exercising his right to a fair legal process.
Incidentally, it was Mr Zuma who appointed Mr Zondo to the role, shortly before he was forced to step down as president.
At the time he said: "I would like to emphasise that I have faith in all the judges and their ability to execute their tasks with the requisite levels of fairness, impartiality and independence."
It would seem those days are long gone.
Mr Zondo has chaired the commission for more than two years and it has heard testimony from ministers, ex-ministers, government officials, politicians and business executives.
Last week Mr Zuma said he was too ill to testify.
As a result, Mr Zondo held a televised media briefing where he ordered Mr Zuma to testify in November.
Making a televised announcement "attests to the fact that he seeks to portray him as uncooperative and belligerent in the eyes of the public", Mr Zuma's lawyer argued in the letter.
The former president first appeared at the commission in July 2019 when he said he had been "vilified".
He was then called back to give further evidence.