Zaid Ibrahim has called on Prime Minister Najib Abdul Razak to accept Dr Mahathir Mohamad’s challenge to a televised debate.

"Why is Najib afraid to have a debate with Tun M (Mahathir)?" asked the former law minister.

"I thought he was always relaxed and confident of his case. That's what he told (former deputy prime minister) Tun Musa (Hitam) and others close to him.

"So lets us ordinary Malaysians hear the PM rebut Tun M piece-by-piece," he tweeted.

In his book titled ‘Frankly Speaking’ launched yesterday, Musa said he was among the first to ask Najib to go on leave in order to facilitate investigations when news of the 1MDB imbroglio broke.

A week later, Musa said, the prime minister, in a telephone conversation, somewhat teased him, saying, "I am still not on leave."

The former deputy prime minister said he had the impression that Najib was in control of the situation and expressed confidence of overcoming the crisis.

Meanwhile, Zaid also commented on Najib's latest salvo against Mahathir, where he also noted how the former premier, like himself, was accused of numerous misdeeds during his tenure.

"Without the debate, Najib can say anything he wants about Mahathir, but he will always be remembered as the PM with billions in his account," he said.

In a blog post today, Najib said Mahathir was accused of corruption, abuse of power and even as the "head of thieves" by those with whom Mahathir is collaborating with now.

On Wednesday, Mahathir, who has been hospitalised for chest infection, challenged Najib to a televised debate about governance in Malaysia, following the latter’s remarks accusing him of trying to boss the prime minister around.

"I am old now. But Allah be praised, I am not senile. I will gladly accept an open debate with Najib with live audience and on live telecast. The debate should be on governing a country.

"I may lose. Cash may win. It is for Malaysians to evaluate, not TV3 or the NST or my own known supporters," said the 91-year-old former premier in a blog post.