The King of Norway, Harald V, has been admitted to hospital and placed on sick leave for two weeks, with Crown Prince Haakon taking over as regent in the meantime, according to a statement from the royal palace.
It said the monarch, 89, has been battling haemolytic anaemia, a blood disorder characterised by the premature destruction of red blood cells.
"In recent weeks, the King has been treated with cortisone," the palace's statement reads. "This has led to fluid retention in the body, which requires hospital treatment."
Harald has been King of Norway since 17 January 1991, following the death of his father, King Olav. Harald married his wife, Queen Sonja, in 1968.
The Norwegian royals have been dealing with a number of health problems in recent months, with Princess Astrid, Harald's sister, admitted to hospital in May with heart failure.
Queen Sonja was also hospitalised in May with heart problems, while Crown Princess Mette-Marit underwent a successful lung transplant in June. Mette-Marit had been diagnosed in 2018 with a rare form of pulmonary fibrosis, a condition that causes lung scarring and difficulty breathing. She was discharged from hospital in mid-July but is not expected to be involved in any official engagements over the next six months as she recovers.
The family has also had to face the arrest and sentencing of Mette-Marit's son, Marius Borg Høiby.
Høiby was sentenced to four years in prison in June on two counts of rape and other offences.
