Former Prime Minister Dr Mahathir Mohamad has been admitted to hospital for bypass surgery after suffering a heart attack in May, the hospital said today.

The 81-year-old will undergo surgery on Tuesday at Kuala Lumpur's National Heart Institute, where he was admitted on Sunday.

His son Mokhzani Mahathir told the New Straits Times newspaper that his father had decided to wait for surgery until after Friday's celebrations marking the country's 50th anniversary of independence from Britain.

"He is all right and preparing for the operation," Mokhzani said.

Mahathir led Malaysia for two decades before handing power to the current premier Abdullah Ahmad Badawi in 2003.

He was last seen in public at the independence day party, waving the Malaysian flag.

The former prime minister was rushed to hospital in May after suffering his second heart attack.

Mahathir had a first heart attack, followed by bypass surgery, in 1989. He also had a mild stroke in November last year.

Despite retiring from office, Mahathir remains an influential figure in Malaysian politics, and has become an outspoken critic of his successor Abdullah, whom he has accused of nepotism and corruption.