The palace in Oslo has confirmed that the King had been suffering with difficulty breathing. Covid-19 has been ruled out as a potential cause, NRK reports.
The king, who has been on the throne for 29 years, is now on sick leave and Crown Prince Haakon will take over his duties temporarily, the palace said in a brief statement.
A keen yachtsman who has taken part in regattas including the 1964 Olympic Games in Tokyo, King Harald and is seen by Norwegians as a unifying figure.
He has insisted he will not abdicate despite declining health that saw him spend a week in hospital in January after a bout of dizziness.
In the early 2000s, he underwent surgery for bladder cancer and later to repair a heart valve.
Harald became king on January 17, 1991 after the death of his father.
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