Former prime minister Dr Mahathir Mohamad has refused to admit or apologise for allegedly laying the foundation that allowed the current political crisis surrounding Prime Minister Najib Abdul Razak to occur.

"Tell me what I did," Mahathir asked while denying criticism that he, among others, undermined the judiciary.

"I don't accept this accusation that I undermined the judiciary.

"The judiciary during my time could even find Umno illegal," Mahathir said.

He added that this would not be possible now, alleging attorney-general Mohamed Apandi Ali would not allow the case to go to court.

The episode in which Mahathir was said to have undermined the judiciary took place in 1988 after Umno was declared illegal by the courts.

This, along with several other decisions made by the courts which did not favour the government, prompted Mahathir to amend the federal constitution to limit the judiciary's powers.

Meanwhile, Mahathir, who quit Umno today, said he did not leave the party because he was upset that his son Mukhriz was not made prime minister.

"If I wanted that, when I was premier, I would have promoted him," he said.

Mahathir was well known for not allowing his kin to be in politics or government-linked businesses when he was still in power.

"I don't practise nepotism," he said.