BANGKOK (REUTERS) - Thailand's Princess Srirasmi, third wife of Crown Prince Vajiralongkorn, has resigned from her royal post, a palace statement said on Friday.

The announcement appeared in the Royal Gazette and follows a high-level corruption investigation that has embroiled several senior police, military officials and one of Thailand's richest men and has implicated relatives of Princess Srirasmi.

Crown Prince Vajiralongkorn asked the government to strip his wife's family of their special royally-bestowed surname in November, after seven of her close relatives were arrested for alleged graft, with charges ranging from bribery to defaming the monarchy.

A palace statement released late on Friday said Princess Srirasmi had decided to give up her royal status.

"Princess Srirasmi, wife of Crown Prince Maha Vajiralongkorn... has written to respectfully resign from her status in the royal family and permission has been granted by His Majesty the King," read the statement.

No reason was given for her resignation.

Princess Srirasmi will lose her royal title, "Mom", which translates as princess.

The news comes at a time of heightened anxiety in Thailand over the health of the revered but ailing King Bhumibol, 87, and nervousness about what a royal succession might bring.

The King was admitted to hospital in October and underwent an operation to remove his gallbladder.

King Bhumibol, seen by many Thais as a unifying figure, was last seen in public in November.

The designated heir to the throne is Crown Prince Vajiralongkorn, a fighter pilot who has in recent years assumed many of his father's ceremonial duties.

The Crown Prince married Princess Srirasmi in 2001.

Thailand's laws protecting the monarchy are among the strictest in the world.

It is a crime to defame, insult or threaten the King, Queen, heir-apparent or regent and anyone convicted of doing so faces up to 15 years in prison for each offence.