The National Audit Office (NAO) has today criticised the government for its use of two different approaches for measuring UK greenhouse gas emissions, claiming that one of the metrics used contradicts government claims that emissions have fallen.

In a new report into the measurement and reporting of UK greenhouse gas emissions, the NAO claims that there is "insufficient consistency and coordination" in the government's measurements and warns that emissions may be 12 per cent higher than officially stated.

Using its most widely quoted metric, UK climate change emissions have fallen over 16 per cent since 1990 to 656m tonnes of CO2 in 2005 putting the economy on course to comfortably exceed its Kyoto imposed emissions target.

But the NAO report found that according to alternative government figures emissions which include emissions from aviation and shipping UK CO2 emissions in 2005 in fact stood at around 733m tonnes.

"There are two different bases on which the government reports emissions: that required for the UN, and the environmental accounts prepared for the Office of National Statistics ... [which are] more comprehensive as they include aviation and shipping emissions," the report explained. "They present UK progress in reducing emissions in a markedly different light."

The report found that the presence of two separate metrics had given government departments "considerable scope" for "aggregating and presenting data in different ways", allowing them to use different figures for different purposes.

The 40 page report will further fuel accusations from environmental groups that the government is fiddling emissions figures by failing to include emissions from aviation and shipping.

The government has sought to tackle concerns about the accuracy of emissions reporting and has instructed its new climate change committee to investigate whether emissions from international aviation and shipping should be included in official emissions reduction targets.

Quoted in The Guardian newspaper, Conservative shadow environment secretary Peter Ainsworth insisted there was an urgent need for a more credible approach to emissions reporting. "This report raises profound questions about the credibility of the government's approach to reducing carbon emissions," he said. "In the absence of reliable and honest reporting the results could be potentially disastrous."