BANGKOK, Thailand - Thailand's ailing king failed to deliver his traditional speech to the nation Thursday on the eve of his 81st birthday because of ill health.

Crown Prince Vajiralongkorn went on national radio to inform Thais that his father, King Bhumibol Adulyadej, was unable to deliver the speech "because he was a little sick."

The king's speech is usually delivered in front of senior government officials and other specially invited guests who represent different sectors of society. It is broadcast live on national radio, and later on television.

After a one-hour delay, the radio announced that Vajiralongkorn will take the king's place at the event. When the prince arrived, he spoke only for three minutes to inform the nation of his father's inability to come.

It is the first time in memory that the longest serving monarch in the world, who has reigned for the last 62 years, has not delivered the speech.

"The king has said to thank (you) for the wishes given out of loyalty. He wants to return the good wishes. He wants everyone to have strong mental and physical health to perform their duties for the public," Vajiralongkorn said.

Immediately afterward, his sister, Princess Sirindorn, addressed the nation to say the king was "eating fine" on Wednesday but has a "lot of phlegm."

However, he has no fever and needed to regain strength before coming out in public.

"His condition is not serious. He is weak," she said.

Many Thais were eagerly awaiting the king's speech, hoping to receive guidance on how to end the country's political crisis caused by a radical group's campaign to change the way governments are elected in the country.

The group occupied the country's airports for one week, but retreated on Wednesday after a court ousted Prime Minister Somchai Wongsawat on Tuesday for voter fraud in the last elections.