KUALA LUMPUR - Malaysia’s longest-serving former premier Mahathir Mohamad is undergoing several medical investigations at the Institut Jantung Negara (IJN), or National Heart Institute, where he was admitted on Thursday evening. 

Tun Dr Mahathir “is currently undergoing several investigations which shall be completed in the next few days”, the IJN said in a brief statement issued on Friday (Dec 17). 

"He shall remain in IJN till then," it said, adding that no visitors were allowed.

No further details were given.

The nonagenarian - once the world's oldest prime minister - was admitted to IJN in Kuala Lumpur at 7pm on Thursday.

"He shall be undergoing a full medical check-up and further observation," the institute said late on Thursday.

Dr Mahathir, 96, has a history of heart ailments. He has suffered three heart attacks, one in 1989 and another two in 2006.

He has also undergone a quadruple bypass surgery. He has periodically been admitted to IJN for medical check-ups over the years.

Dr Mahathir served nearly 24 years in total as Malaysia's prime minister over two separate terms, leading the Barisan Nasional and Pakatan Harapan (PH) administrations.

He last served as premier from 2018 to early 2020, when the PH government he was leading collapsed due to his resignation and mass defections from his then-party, Parti Pribumi Bersatu Malaysia.

He is currently MP for the Langkawi constituency in his home state of Kedah, and chairman of opposition Parti Pejuang Tanah Air Malaysia.