The findings, still the subject of scientific debate, could destabilise the European Union's fledgling carbon trading system and have implications for the Kyoto treaty.

"Work at the (European Commission's) Joint Research Centre (JRC) in Italy suggests huge under-reporting of many national CH4 (methane) emissions," said Euan Nisbet of London's Royal Holloway University.

"Top-down science is still somewhat in its infancy. But the gas they measure is there, not an estimate of what they think should be there," he told Reuters.

According to work by Peter Bergamaschi at the JRC in Ispra, Italy, top-down science suggests that Britain may be reporting only half its actual methane emissions and France only two-thirds, the magazine New Scientist said on Wednesday.

By contrast, Ireland and Finland may be over-reporting the methane coming from their peat bogs.

Britain defended its estimates on Wednesday, saying they were calculated in line with international guidelines reviewed each year by independent international experts.

The government's Department of Environment, Food and Rural Affairs (DEFRA) said in a statement that it believed Bergamaschi overestimated British methane emissions by at least half.

"Bergamaschi's work cannot separate natural methane emissions from man-made ones. There is significant uncertainty in how much natural methane is produced in the UK, which is carried into Bergamaschi's model," DEFRA said.

Nisbet said making the same calculations for carbon dioxide, more plentiful but less damaging, was more complicated.

The world needed a chain of monitoring stations, similar to the seismic system set up in the 1950s to monitor nuclear bomb tests, he said.

Nisbet said China, which is building a coal-fired power station a week to fuel its booming economy, had good monitoring as had Canada, as well as Kyoto refuseniks the United States and Australia.