The true scale of climate change emissions from shipping is almost three times higher than previously believed, according to a leaked UN study.

It calculates that annual emissions from the world’s merchant fleet have already reached 1.12bn tonnes of CO2, or nearly 4.5% of all global emissions of the main greenhouse gas.

The report suggests that shipping emissions - which are not taken into account by European targets for cutting global warming - will become one of the largest single sources of manmade CO² after cars, housing, agriculture and industry.

Until now, the UN’s Intergovernmental Panel on Climate Change has estimated shipping emissions to be a maximum 400m tonnes, but the new draft report shows emissions are much worse than feared, and warns CO2 emissions are set to rise by a further 30% by 2020.

Contacted about the contents of the report, Dr Rajendra Pachauri, chair of the IPCC, said: “This is a clear failure of the system. The shipping industry has so far escaped publicity. It has been left out of the climate change discussion. I hope [shipping emissions] will be included in the next UN agreement. It would be a cop-out if it was not. It tells me that we have been ineffective at tackling climate change so far.”