“I think if it is not true, the prime minister should take action, should sue them for false reporting.

“If the PM doesn’t take any action, then it must be because there is some truth in what has been reported,” said the former premier in a press conference at the Proton Centre of Excellence here today.

He was commenting on WSJ’s latest exposé, claiming that global investigators had found that over USD1 billion had been channeled into Najib’s accounts, an amount hundreds of millions more than was previously identified and believed to have originated partly from state investment firm 1Malaysia Development Berhad (1MDB).

The US-based business daily published its first exposé of Najib’s alleged misappropriation of funds in July last year and although the prime minister has denied all allegations, he has yet to take any legal action against the publication.

Dr Mahathir in a surprise announcement yesterday, said one of the reasons he decided to leave Umno was because he was ashamed of the international scandals Najib, as party president, was currently implicated in.