There were no injuries to the five other crew on the 90-meter (300-foot) vessel, which was carrying ferrous metal from Denmark to Portugal, police said.
Fire crews went to the vessel after it docked in Dover, southern England, on Friday. A team of chemical specialists was sent to check the ship, the Kent Fire and Rescue service said. Police, who are investigating the deaths, said there was no risk to the public from the docked ship.