Former premier Mahathir Mohamad's office on today quashed rumours that he was seriously ill, saying he had been admitted to hospital for a routine cardiac check-up.

"It's routine, as in it's every year. Unfortunately this time around it has spread some rumours," his spokesman told AFP after the 80-year-old checked into the National Heart Institute late yesterday.

"The rumours about him having another bypass are not true," he said, adding he was expecting to be discharged tomorrow. "The fact that he'll be discharged means he's OK."

Mahathir underwent a heart bypass operation in 1989.

After more than two decades in power, he stepped down two years ago in favour of current Prime Minister Abdullah Ahmad Badawi, but maintains a high profile with frequent speeches and press conferences.