The government is facing fresh calls to include shipping in its carbon emission targets after a leaked UN report today revealed that the sector's carbon footprint is almost three times higher than previously believed.

According to reports in The Guardian, the study from the International Maritime Organisation (IMO) estimates that annual emissions from the world's maritime fleet have reached 1.12bn tonnes, or almost 4.5 per cent of all global carbon emissions. It also warns that predicted growth in world trade means emissions from the sector could climb by a further 30 per cent by 2020, making shipping one of the world's largest emitters.

Until now, official UN figures from the Intergovernmental Panel on Climate Change (IPCC) estimated that shipping accounted for 400m tonnes of carbon emissions, equating to around two per cent of global emissions and putting the industry roughly on a par with the aviation sector. However, the latest figures are considered more accurate because they rely on engine size data, combined with the time ships are at sea and the amount of low grade fuel sold to shipowners.

Jeremy Harrison, a spokesman for the British Chamber of Shipping, said that the IMO findings were "not surprising" and that the IPCC estimates had always been far too low.

He also argued that simple comparisons with aviation industry emissions were erroneous given the larger scale of the shipping sector. "Shipping is on a different order of magnitude to aviation," he said. "Comparing overall emissions is not appropriate, what is appropriate is comparing emissions per tonne of material transported per km. When you consider that figure the shipping industry is incredibly carbon efficient."

According to British Chamber of Shipping figures the average merchant ship emits five kg of CO2 per tonne per km, compared to 50kg for road freight and 540kg for a tonne of goods transported by Boeing 747. "For any movement of goods shipping is more efficient than any other form of transport," argued Harrison.

But regardless of shipping's relatively high levels of fuel efficiency the report's findings will still increase pressure on the government to include shipping in the UK's emission targets – something Gordon Brown has already said the new independent Climate Change Committee will investigate.

Friends of the Earth head of campaigns Mike Childs said that the government now had little choice but to account for shipping in its carbon reporting and new climate change law. "It's ludicrous to leave them out," he said. "A bit like introducing a drink driving law that discounts whisky."

The IMO report will also lead to renewed calls for shipping to be included in the EU's emissions trading scheme, a fate that awaits the aviation sector from 2011.

Harrison said it was too early to tell if the shipping industry would support inclusion in the cap-and-trade scheme, but warned that it could prove difficult to make any expanded scheme work without penalising shipping nations. "A country like Denmark generates more CO2 from shipping than the rest of its economy combined," he explained. "Any attempt to bring shipping into a trading scheme would have to account for that fact."

He also warned that shipping will find it harder to deliver further reductions in carbon emissions than other sectors due to its already high levels of carbon efficiency. "Because of high fuel costs ship operators are already looking at ways to improve efficiency, such as improved hull designs, new fuel types and even operating at slower speeds," he said. "But because the industry is already a very efficient way of moving goods it will be difficult to deliver large percentage improvements."