German Chancellor Angela Merkel was second, with some 7,400 tonnes for 174,440 kilometres over the course of 75 trips, most of which (44) were within Germany.

French President Nicolas Sarkozy, who traveled 324,595 kilometres, or the equivalent of flying eight times around the Earth, produced 7,100 tonnes of CO2.

Spanish Prime Minister Jose Luis Rodriguez Zapatero came fourth, with 6,700 tonnes of CO2 for 175,495 kilometres.

Terra Eco based its emissions calculations on the itinerary of EU leaders and the type of aircraft used.

It says Mr Brown's flights were the most polluting ones because he flies in larger or less fuel-efficient aircraft.

Downing Street did not disclose what type of plane the premier flies in because of security reasons, but based on information from journalists travelling with him and aviation experts, the magazine made its calculations believing he uses a series of Boeings: 747, 757 and 777.

Mr Sarkozy flew in an Airbus A319, Ms Merkel in an A310 and Mr Zapatero in an A310 and a Super Puma helicopter, which he used for a third of his 48 trips.

The EU sees itself as the world leader on environmental issues, and the bloc's member states last December reaffirmed their commitment to cut carbon emissions and improve energy efficiency by 20 percent, both by 2020, in a bid to fight global warming.