Britain's climate change emissions may be 12% higher than officially stated, according to a National Audit Office investigation which has strongly criticised the government for using two different carbon accounting systems. There is "insufficient consistency and coordination" in the government's approach, the NAO said.

Using one system, which the government presents to the UN and in public, Britain emitted 656m tonnes of CO2 in 2005, and claims an improvement on 1990 figures. However, the lesser-known but more accurate data in the government's national environmental accounts show emissions to be in the region of 733m tonnes in 2005, a NAO report says today.

"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. They present UK progress in reducing emissions in a markedly different light", says the report.

The report says there have been "no reductions in UK emissions" if measured by the national accounts method.

The figures contained in the report fly in the face of consistent government claims that it is reducing emissions. Last week the environment minister, Phil Woolas, said in a Commons written answer: "UK greenhouse gas emissions have fallen by 16.4% since 1990. We remain on course to nearly double our Kyoto Protocol target over the 2008-12 period."

Last night opposition parties and environment groups accused the government of misleading the public at a time when the UK claims to be leading the world in achieving reductions. "This report raises profound questions about the credibility of the government's approach to reducing carbon emissions. In the absence of reliable and honest reporting the results could be potentially disastrous", said Peter Ainsworth, shadow secretary of state for the environment.

"Labour's claim that Britain's carbon footprint is shrinking is a scam. The figures on aviation are being fiddled, meaning the government can give the green light to airport expansion without most of the subsequent rise in emissions being counted", said Robin Oakley, of Greenpeace.

The 40-page NAO report found that government departments interpreted data in different contexts and for different purposes, "in one case within the same document on successive pages".

The report said the government's many targets and timetables for reducing different combinations of greenhouse gases were confusing. The UK has a Kyoto target of 12.5% reduction in all greenhouse gases by 2012, an EU target of reducing 20-30% of CO2, three domestic goals ranging between 20-60% of CO2 and is in the process of drafting a new climate change bill .

"The targets can be assessed against different bases. There is "considerable scope" for aggregating and presenting data in different ways", the NAO said.