Former premier Dr Mahathir Mohamad is in a stable condition after being moved back into intensive care following heart bypass surgery, said his daughter.

Mahathir spent just a day in a normal ward before being shifted into the intensive care unit Friday, but his aides said his condition had not deteriorated.

"His condition is stable. The doctors say Tun will remain longer in the ICU due to the need to prevent any infection," his daughter Marina Mahathir told the official Bernama news agency, using his honorific title.

"The doctors have imposed no restriction on Tun's food to enable him to gain strength," she said.

Mahathir underwent the five-hour operation on Sept 4 after suffering two heart attacks within a year. He previously had bypass surgery in 1989.

The 82-year-old former leader resigned in 2003 after 22 years in power, handing the reins over to the current prime minister, Abdullah Ahmad Badawi.

Despite retiring from office, Mahathir remains a hugely influential figure and has become an outspoken critic of his successor, accusing him of economic mismanagement, nepotism and corruption.