Bangkok - Thai King Bhumibol Adulyadej, 81, has suffered from a soar throat and slight fever for several days, media reports said Saturday citing the Royal Household Bureau.

A statement issued by the bureau late Friday said the king had suffered a sore throat since Wednesday, forcing doctors to put him on a liquid diet, and had a slight fever.

"A blood test on Wednesday showed evidence of infection, but found no trace of bacteria in blood or sputum samples," said the bureau's statement.

King Bhumibol's illness was cited as the reason for his failure to deliver his annual birthday-eve speech on Thursday.

Crown Prince Maha Vajiralongkorn and Princess Maha Chakri Sirindhorn informed more than 22,000 government officials, bureaucrats, dignitaries and common people who had gathered at the Chitralada Palace Thursday night to hear the king's address of his cancellation of the much-awaited event due to a slight illness.

It was the first time the king had missed the annual event in decades.

His past speeches have touched on Thailand's political, social, cultural and development challenges, providing rare glimpses of the king's opinions on current events and trends.

As the head of Thailand's constitutional monarchy, King Bhumibol has few formal political powers but has amassed considerable moral authority over the past six decades of his rule.

He has dedicated much of his 62 years on the Thai thrown to development projects for the rural poor, although his active schedule has been slowed down by illnesses over the past decade.

At 81, King Bhumibol is the world's longest-reigning monarch. (dpa)