The Swedish coastguard is searching for three containers filled with hazardous materials which have fallen overboard from a cargo ship.
The accident happened when the bottom container in a stack of four collapsed, toppling the others into the water.
One of the missing containers is packed with five tonnes of flammable substances and the other two are holding materials hazardous to marine life.
The Finnish owned vessel, Linda, was on its way from Rotterdam to St Petersburg when the containers went into the water south of Gotland in the Baltic Sea.
All sea traffic in the area has been alerted. If they remain semi-submerged, the containers could present a danger to shipping.
Finnish marine accident investigators are planning to question the ship’s skipper when the Linda reaches its destination.