Former premier Dr Mahathir Mohamad was admitted to the National Heart Institute early Sunday for a medical check-up after being hit by a bout of flu.

The ex-prime minister was taken in for a "routine observation and blood tests since this morning," his aide Sufi Yusoff said in a statement.

"Mahathir is expected to be kept under observation for the next few days pending doctors' advice," he added.

The 81-year-old premier was admitted to the heart centre last November after suffering a mild heart attack. He previously suffered a heart attack in 1989 and had bypass surgery.

Renews attack

On Saturday, Mahathir renewed his attack against his successor Prime Minister Abdullah Ahmad Badawi's administration, saying members of the ruling Umno must voice their views without fear.

"We have to be bold enough to criticise leaders if their decisions are not good and undermine the self-esteem of the people," he was quoted as saying by Bernama news agency in the southern state of Johor.

The former prime minister handed power to Abdullah in October 2003.

The two are embroiled in a bitter dispute, with Mahathir accusing Abdullah of economic mismanagement, corruption and nepotism - charges the incumbent leader has repeatedly denied.