Princess Mary's father-in-law, Prince Consort Henrik, is suffering from dementia.
A statement released by the palace read: 'Following a longer course of investigation, and most recently, a series of examinations conducted during late summer, a team of specialists at Rigshospitalet has now concluded that His Royal Highness Prince Henrik suffers from dementia.
'The extent of the cognitive failure is, according to Rigshospitalet, greater than expected considering the age of the prince.'
It is expected the 83-year-old's royal duties will be scaled back as a result of the diagnosis.
The news comes just weeks after it was revealed the prince did not want to be buried next to his wife, Queen Margrethe, because she did not make him King.
'I think she’s let me down. I think we are doing well together, but every man cannot accept that he is not on par with his spouse,' Henrik says.
'My wife has decided that she wants to be queen, and I’m very happy about that. But as a human being she needs to know that if a man and wife are married, they are equal,' Henrik says of the Queen, who became queen in 1972 after the death of her father.
'It’s her who makes me a fool,' Henrik added. 'I have not married the Queen to be buried in Roskilde.'