BANGKOK, April 9 (Reuters) - Army-appointed Thai Prime Minister Surayud Chulanont was admitted to a Bangkok hospital on Monday for a four-day medical check-up, his doctor said, amid rumours he would resign.

"Overall his health is fine, but we have to check his intestine system thoroughly in the next couple of days," Prinya Taweechaikarn told reporters.

Surayud, appointed by the military after its Sept. 19 coup against Prime Minister Thaksin Shinawatra, was expected to remain in the private Bangkok Hospital until Thursday, Prinya added.

Initial results had revealed nothing unusual, he said.

The former army chief abruptly cancelled all his engagements on Monday, fuelling speculation he might be stepping down from the year-long job which he has openly said he was not enjoying, but which he has vowed to see out to the end.

Government spokesman Yongyuth Mayalarp denied the resignation rumours, telling a news conference Surayud had cancelled his appointments as he had "personal business to attend to", the Thai News Agency reported.

Surayud's term is due to end once a new constitution has been drawn up and elections have been held. He has promised polls in December.

Opposition to the coup -- the 18th in Thailand's 75 years of on-off democracy -- has risen in the last month, with several protest groups and web sites targeting Surayud, army head Sonthi Boonyaratglin and chief royal adviser Prem Tinsulanond.

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