Global emissions of carbon dioxide from shipping are twice the level of aviation, one of the maritime industry's key bodies has said.
A report prepared by Intertanko, which represents the majority of the world's tanker operators, says emissions have risen sharply in the past six years.
Previous International Maritime Organisation estimates suggested levels were comparable with those of planes.
Some 90,000 ships from tankers to small freighters ply the world's oceans.
Intertanko says its figures are the most realistic estimation of the current levels of CO2 from ships.
It says that growth in global trade coupled with ships burning more fuel to deliver freight faster has contributed significantly to the increase.
Dragos Routa, the technical director of Intertanko, told the BBC the figures were a work in progress but the levels of emissions had risen sharply.
While there are few accurate measures and even fewer restrictions on the amounts of carbon dioxide that ships can emit at present, governments in many parts of the world are considering a clampdown as part of their efforts to tackle global warming.