Outspoken former prime minister Dr Mahathir Mohamad suffered a heart attack before he was hospitalised earlier in the week, doctors said.

The 81-year-old was taken in after having breathing difficulties and the National Heart Institute, where the former premier is now being treated, said he had had a heart attack but was making "steady positive progress".

"The difficulty of breathing was caused by lung congestion associated with myocardial infarction," the institute said in a statement.

Mahathir previously suffered a heart attack in 1989 and had bypass surgery. He also had a mild stroke in November last year, caused by a clot that restricted blood flow in one of his arteries, but subsequently recovered.

Not allowed to see visitors

One of Mahathir's sons, Mukhriz, said his father was making significant improvement after the heart attack - the fourth of such attacks.

"The breathing difficulties were a direct result of problems he had with the lungs which was also due to some heart difficulties," said Mukhriz.

"At the moment we are seeing significant improvements as the days go by," he said. "I wouldn't say he's fully recovered, but it sure seems he's on the right path towards recovery."

Mukhriz also said that, after a full recovery, Mahathir and family members would discuss his father's "pace of activity and lifestyle. As recently as last week he was going horse riding and all that."

He said Mahathir was not allowed to see any visitors, but that his wife Siti Hasmah Mohamad Ali, was receiving well-wishers at the institute.