Prime Minister Surayud Chulanont has been admitted to hospital for three days to undergo a "normal physical check-up", a doctor said yesterday.

Maj General Dr Parinya Thawichaikarn, a personal doctor to Surayud, organised a press conference in the afternoon after the news that Surayud was admitted to Bangkok Hospital brought speculation that he will step down.

Parinya said Surayud had postponed his physical check-up for more than a year because of his tight work schedule after assuming the premiership.

Parinya dismissed speculation that the PM was under heavy stress and was disheartened by rising political pressure and will step down, adding Surayud showed no signs of being under stress because he always meditates.

 "I have known him for over eight years. He has a strong commitment, especially if the commitment concerns the national interest.

"He gives the country the first priority before his own life. There is no way he will surrender,'' Parinya said.

The doctor said he needed to thoroughly check Surayud, especially his intestines, because eight years ago he was diagnosed with an infection.

He said he made an appointment with Surayud on Sunday, as the timing for the check-up was suitable because of the long Songkran holiday. If he needed to undergo any medical treatment, it would done during the holiday.

Parinya explained he needed about three days as Surayud is to be placed on a strict diet and administered with a laxative for two days so that his colon is clean and his intestine is ready to undergo a  "virtual colonoscopy".

He said the PM had already undergone blood tests and a cardiac check, which showed his heart was as strong "as men in their 40s" while his blood showed he had acceptable levels of calcium.

If the tests go well, the PM will be released from hospital on Thursday.