South Africa's former president Jacob Zuma did not show up on Monday at an inquiry into corruption during his time in office despite the country's top court ordering him to appear.

The inquiry led by Deputy Chief Justice Raymond Zondo is probing allegations of high-level graft during Zuma's nine years in power from 2009 to 2018.

Zuma denies any wrongdoing but has not cooperated with the so-called "state capture" inquiry. He was removed from office by his governing African National Congress (ANC) in February 2018, in a move orchestrated by allies of his successor Cyril Ramaphosa.

Zuma's lawyers confirmed in a letter to the inquiry that the former president, who is now 78, would not attend this week, when he had been due to give evidence.

"The summons issued for our client to appear on Feb 15 to 19 2021 is irregular," the letter said, justifying his no-show.

Ramaphosa has been trying to clean up the ANC's image and restore investor confidence in Africa's most industrialised nation since Zuma's departure. However, he has faced opposition from a faction in the ANC that remains loyal to Zuma.

The allegations against Zuma include that he allowed businessmen close to him - three brothers Atul, Ajay and Rajesh Gupta - to plunder state resources and influence policy.

The Guptas deny any wrongdoing. They left South Africa after Zuma was ousted.

WALKOUT

Zuma walked out of the inquiry in November without permission. Its officials then approached the constitutional court to make him re-appear and testify.

"It's difficult to understand why he (Zuma) would be scared of taking the witness stand and subjecting himself to questioning like everyone else," Zondo said on Monday.

One of Zuma's lawyers did not respond to a message from Reuters seeking comment.

A lawyer for the inquiry, Paul Pretorius, said Zuma had been implicated by the evidence of at least 40 witnesses.

"Whether Mr Zuma believes he has been accused of wrongdoing or not … his responses to those allegations are still directly relevant to the work of this commission," Pretorius said.

Speaking near Zuma's Nkandla homestead, Edward Zuma, one of his sons, told reporters there had been a longstanding conspiracy against his father and that the inquiry had treated him differently to other witnesses.