Former president Jacob Zuma has been granted leave to appeal the Pretoria high court ruling that the decision to place him on medical parole was invalid and unlawful.

The court had last week ordered that Zuma return to prison to serve the remainder of his 15-month sentence and that time spent outside prison should not be taken into account.

Zuma's lawyers successfully argued before judge Elias Matojane on Tuesday that another court could reach a different outcome.

Matojane ruled that Zuma's lawyers have put compelling grounds that they have prospects of success in challenging his order.

He said the case "raises important questions of public law arising from the interpretation and application of Section 75 sub-section 7 read with Section 79 of the Correctional Services Act 111 of 1998 and regulations promulgated thereunder."

"In my view this matter merits the supreme court's attention for certainty and the correct interpretation and application of these provisions. I am also of the view that there's a reasonable possibility that another court may find that the order that (Zuma's) time spent on medical parole should not count towards fulfilling his sentence as this impacts on him unfairly," Matojane said.

He said it was correct that it was not Zuma's own decision to place himself on medical parole, as argued by Zuma's lawyer Adv Dali Mpofu earlier.

"His meeting with his political allies and a prayer meeting was a once off thing, his freedom of movement is restricted and this has an impact on his dignity. He's serving his sentence albeit outside of prison," Matojane said.

He said because of Zuma's illness and advanced age, "he needs compassion, empathy and humanness which is the essence of Ubuntu. In my view, another court might take this into consideration and come to a different ruling.

"For the above reason, leave to appeal is granted to the Supreme Court of Appeal," said Matojane.