According to Monday's announcement, the queen had a fever and coughed more than usual early on Sunday so doctors invited her for a check-up at a hospital in Bangkok. She was then diagnosed with influenza and is being treated accordingly.

Queen consort to late king Bhumibol Adulyadej, who died in 2016 after seven decades on the throne, and mother of 66-year-old King Maha Vajiralongkorn, Sirikit turned 86 last week.

She did not address the public on her birthday and she has not made any public appearance in recent years due to her deteriorating health.

However, the palace released photos of the queen celebrating her birthday privately with her children at a Bangkok palace, showing her in a wheelchair, looking frail and with very short white hair.

Sirikit is revered as the nation's mother figure. Her birthday has also been celebrated as Thailand's Mother's Day. She is to remain at the hospital until further notice, according to the palace.

Her husband's death triggered a one-year official mourning period.

Regarded as Thailand's father figure and moral compass, Bhumibol was cremated in October in an elaborate five-day ceremony costing up to $US90 million.

Australian Associated Press