PUTRAJAYA - Malaysia's Prime Minister Mahathir Mohamad on Monday (May 6) called Johor's crown prince "a little boy" and "stupid", harsh words that are likely to escalate the row between the elder statesman and the Johor royal family.

The comments made by Tun Dr Mahathir followed weeks of angry exchanges between him, the prince Tunku Ismail Sultan Ibrahim, and the Johor ruler Sultan Ibrahim Sultan Iskandar.

They had argued back and forth on whether under the Constitution, the federal government or the royal family has the ultimate right to administer the state government and appoint top state officials.

Speaking in a media interview in Putrajaya, Dr Mahathir, 93, said Tunku Ismail, 34, should stop commenting on matters beyond his knowledge, Malaysiakini reported Dr Mahathir as saying.

Tunku Ismail is widely referred to in Johor as the TMJ, short form for Tunku Mahkota Johor or Johor's crown prince.

"I don't want to comment on the sultan because if I say anything that is not good, it's not nice because he's the sultan," Dr Mahathir was quoted as saying.

"But this TMJ (Tunku Ismail) is a little boy… He is stupid because he doesn't know what is happening. So don't talk. When you don't know anything, don't talk," the premier said.

The heated debate between Dr Mahathir and the Johor royals is followed closely in Malaysia amid heightened tensions between the year-old Pakatan Harapan (PH) government and the main Malay opposition parties, Umno and Parti Islam SeMalaysia (PAS).

Umno and PAS have accused PH of diluting Malay and Muslim rights since it came to power and of eroding the strength of the nine Malay royal houses and the institution of the Malay monarch.

The debate also goes to the heart of the pitched battle between PH and the opposition parties for the support of the Malay majority, who made up 60 per cent of the country's 32 million population.

PH won about 30 per cent of the Malay vote at the May 2018 general election, and some 95 per cent of the non-Malay vote.

Pollsters have indicated that PH's grip of the Malay vote has been slipping, as Malays are worried about the dilution of their rights, and due to PH's failure to resolve bread and butter issues and fulfil many of their election promises.

Dr Mahathir's remarks on the crown prince was made when he was asked to comment on Tunku Ismail's statement last weekend that his father's good deeds, such as giving land to the people and government, were not highlighted.

Tunku Ismail's lament was made following the surprise revelation that a plot of federal land earmarked for the Johor Baru-Singapore Rapid Transit System (RTS) rail link had been quietly transferred to Sultan Ibrahim.

The ruler had said through his aide that he was not aware of the transfer, and has said he is prepared to surrender the land back to the government without cost if the RTS project is immediately carried out.

On Monday evening, PM Mahathir's communication and media advisor A Kadir Jasin asked new Malaysian police chief Abdul Hamid Bador to "reopen the IP (investigation papers) which were frozen or closed."

Datuk Kadir in his Facebook posting did not explain what the police investigation papers were about.

He also asked the Inland Revenue Board (IRB) to check how much in taxes has Mados Sdn Bhd had paid after being in business for "hundreds of years."

Mados is owned belongs to the Johor royal family, Malaysiakini news said. A record in the Companies Commission of Malaysia said carries out Mados property investment, management services, timber logging and quarry operations.