Shaik told the Hefer Commission last week that he had reason to believe there was an apartheid government agent working in the office of the anti-apartheid lawyer Griffiths Mxenge, who was assassinated by the security police.
This he was told by former security policeman Dirk Coetzee, who was in charge of Mxenge's assassination, Shaik said.
It is well-known that Ngcuka served his articles with Mxenge's Durban attorney firm.
Shaik added that the ANC's intelligence department at the time had reason to believe that Ngcuka did not share Mxenge's "strong views of the ANC". For this, Mxenge allegedly asked him to leave his office.
Ngcuka's counsel, Advocate Marumo Moerane, told Shaik on Monday that his client was "very hurt" about this "innuendo". He was like a son to Mxenge.
Ngcuka had such high regard for Mxenge and his wife, Victoria, that the building in which he worked today was named after them, Moerane said.
Shaik denied that he suggested a correlation between his allegations (from which could be inferred that Ngcuka had sold Mxenge out).
"I apologise unconditionally," he repeated more than once.