A seven-year-old boy from Poland and his mother have tragically died after falling from a ferry boat sailing in the Baltic Sea, as announced by Swedish authorities who are further investigating the case.

The child fell, under currently unclear circumstances, from the Stena Spirit ship with his mother instinctively jumping after him into the sea in an apparent attempt to save him.

According to the first information, the ship was in the middle of its journey which started yesterday from Sweden to Poland.

Terrified passengers watched as rescue helicopters, including one from a nearby NATO unit, circled over the water for about an hour desperately trying to locate the mother and son.

The mother was found in the icy water 59 minutes after the alarm was sounded and her son seven minutes later, according to a Stena Line official.

The bodies were taken by helicopter to Karlskrona Hospital.

Swedish prosecutors said this afternoon that they had opened a preliminary investigation into the case.