Derby-Lewis is one of two men accused of the 1993 murder of SA Communist Party leader Chris Hani.
Derby-Lewis is currently being treated for prostate and skin cancer.
Derby-Lewis has twice been denied ordinary parole.
Seventy-five-year-old Derby-Lewis has served 18 years of his 25 year sentence.
His lawyers have said that the minister Mapisa-Nqakula had withheld reasons for the latest refusal for parole until a Public Protector becomes involved in the case.
Derby-Lewis’ legal representative said he has been trying to get the accused murderer free on parole for over three years. He will only be eligible for parole again after two years.
The minister cited the convict’s lack of full disclosure and lack of remorse for his crime as reasons for his parole being refused.