The 89-year-old monarch has been in hospital in Oslo since 17 August with a blood disorder as senior royals cancel their engagements.
Norway's 89-year-old King Harald is in an "extremely serious" condition after his health deteriorated while being treated in hospital for a blood disorder, the royal palace said on Thursday.
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"His Majesty the King's health condition is extremely serious," the palace said in a statement, without providing further details.
Queen Sonja and Crown Prince Haakon, who is acting as regent during his father's illness, have cancelled all royal engagements, while Norwegian media reported that members of the royal family were at the king's bedside.
Europe's oldest reigning monarch has been in Oslo University Hospital since 17 August suffering from haemolytic anaemia, a condition in which red blood cells are destroyed faster than they can be replaced.
The king was admitted after suffering fluid retention linked to cortisone treatment and has experienced a series of health problems in recent years, including having a pacemaker fitted.
Harald, who has reigned since 1991, has scaled back his official duties but has repeatedly ruled out abdicating, saying the oath he took before Norway's parliament was for life.