On the night of Saturday, a passenger fell on the boat Viking Glory and was picked up by an ambulance helicopter.
The fall accident happened on a staircase and the passenger suffered a serious head injury, according to Pontus Svensson, maritime rescue manager at the Norwegian Maritime Administration.
The ship is said to have turned back towards the Swedish mainland to meet the ambulance helicopter from the Swedish Maritime Administration.
– We received an alarm around midnight. The ambulance helicopter brought the passenger to Norrtälje hospital, he says.
No one else is said to have been injured in the accident.