Britain's true contribution to global warming is much higher than official figures show, ministers admit. And it has been rising rapidly at the very time that they have been boasting that it has been falling.

The admission undermines the Government's claim to be in the vanguard of cutting the pollution that causes climate change – on the eve of negotiations among the world's most powerful leaders this week.

Global warming will be high on the agenda of the G8 summit that opens in Toyako, Japan, tomorrow – along with the rise in oil and food prices, Third World development and Zimbabwe. It will also be the focus of simultaneous talks with the leaders of China, India, Brazil and other large developing countries. Ministers have long presented Britain as a leader in tackling climate change, and last week a report published by WWF (formerly the World Wildlife Fund) and Allianz, the insurance company, concluded that it was performing better than any other G8 country. But figures revealed last week in an obscure government report – snappily entitled Development of an Embedded Carbon Emissions Indicator – tell a much more sobering story.

Produced by the Stockholm Environment Institute and Sydney University for the Department for Environment, Food and Rural Affairs (Defra), it concludes that Britain is responsible for 200 million more tons of carbon dioxide emissions each year than official figures admit, an increase of 37 per cent.