Former premier Dr Mahathir Mohamad is stable and able to breathe unaided after surgery to remove infected tissue from his chest, his family and doctors said today.

Mahathir, 82, suffered a chest infection following his second heart bypass procedure on Sept 4 and underwent the three-hour operation late Saturday.

"Doctors are happy that his vital parameters continue to be stable. Mahathir is also conscious as doctors gradually ease on his support medication," a statement from the National Heart Institute said.

Mahathir's daughter, Marina, said the former premier's condition was improving in several ways.

"Today he is breathing more on his own than yesterday... And he is more awake and according to mum, more alert too," she said in her blog.

"As another positive sign, she says he is displaying signs of impatience. Good old cranky dad!" Marina added.

Two heart attacks

Mahathir has suffered two heart attacks in the past year. He had his first heart bypass operation in 1989.

He resigned in 2003 after 22 years in power, handing power to the current prime minister, Abdullah Ahmad Badawi.

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