Thailand's King Vajiralongkorn has stripped his royal consort of her rank and titles for "misbehaviour and disloyalty against the monarch".
An official announcement said Sineenat Wongvajirapakdi had been "ambitious" and tried to "elevate herself to the same state as the queen".
"The royal consort's behaviours were considered disrespectful," it said.
She was appointed in July, just two months after the king married Queen Suthida, his fourth wife.
Sineenat, who was a major-general and is a trained pilot, nurse and bodyguard, was the first person to be awarded the title of Royal Noble Consort in nearly a century.
Queen Suthida - a 41-year-old former flight attendant and deputy head of his bodyguard unit - is King Vajiralongkorn's long-term partner and has been seen with him in public for many years.
The announcement that Sineenat had been stripped of her titles was published in the Royal Gazette on Monday. It marked a sudden fall from grace for the royal consort, who for several years was seen often at the side of King Vajiralongkorn.
Even after the king's marriage to Queen Suthida, Sineenat was a regular guest at royal events.
What did the palace say?
The statement published on Monday said Sineenat had "shown resistance and pressure in every manner to stop the appointment of the Queen" ahead of the coronation in May.
"The king gave her a royal consort position, in hopes of relieving the pressure and a problem that could affect the monarchy," the statement said.
It also accused the royal consort of "resistance against the king and the queen" and of abusing her power to give orders on the king's behalf.
The king, the statement said, had learnt "she neither was grateful to the title bestowed upon her, nor did she behave appropriately according to her status".
He ordered her stripped of all royal titles, decorations, status in the royal guard and her military ranks.
King Vajiralongkorn ascended the throne after the death of his father in 2016.
What about the king's wives?
He has had four wives - Princess Soamsawali from 1977 to 1993; Yuvadhida Polpraserth from 1994 to 1996; Srirasmi Suwadee between 2001 and 2014; and Queen Suthida.
The true cause of Sineenat's removal as royal consort may never be made public, given the secrecy which cloaks palace affairs in Thailand. The nation's lese-majeste law forbids any insult of the monarchy and is among the strictest in the world.
The removal of Sineenat echoes the cases of two of the King's former wives. In 1996, he denounced his second wife, who fled to the United States, and disowned four sons he had with her.
In 2014, his third wife Srirasmi Suwadee was similarly stripped of all her titles and banished from court. Her 14-year-old son has been raised by King Vajiralongkorn in Germany and Switzerland.
The King has also exercised his royal powers in a more direct way than his recent predecessors. Earlier this month, the two most important army units in the capital, Bangkok were placed directly under his command - a concentration of military power in royal hands unprecedented in modern Thailand.