Thanks to newer fleets, higher densities of seats and direct flights, no-frills airlines produce as little as 35 per cent of the carbon emissions, per passenger, of full-service airlines, according to analysis by Liligo.co.uk, a flight-comparison website.

A couple flying with Ryanair from London to Venice and returning a week later have a carbon footprint of 410kg, while the equivalent journey on Alitalia would produce 977kg. A flight from London to Zurich with easyJet has a carbon footprint of 277kg per couple, compared with 688kg with Aer Lingus.

No-frills airlines tend to fly with more seats occupied: in 2009, the average for an easyJet flight was 86 per cent and for Ryanair 82 per cent. This compares with an average of 68 per cent on Europe’s full-service airlines, according to the Association of European Airlines. British Airways’ flights were 73 per cent full, on average, during the same period.

An easyJet spokesman said: “Our policy is to expand our fleet through the acquisition of the latest-technology aircraft, as these are more fuel-efficient than older models. The average age of an aircraft in our flight is 3.5 years. We also use these aircraft as efficiently as possible, by maximising load factors and seating density.” On an Airbus A319, the average full-service airline has 124 seats; easyJet has 156.

No-frills carriers also operate from smaller “point-to-point” regional airports, which allow shorter taxi times and have fewer delays for landing slots.

“Our analysis shows that the environmental stigma of budget travel may be unwarranted,” said Gbenga Kogbe of Liligo.co.uk. “Travellers can now assess the financial and environmental costs of travelling with low-cost airlines, traditional airlines and charter-flight companies.”

Full-service airlines continue to take steps towards the low-cost model. This week Bmi said it planned to remove business-class sections from its domestic flights and have economy-only cabins. It already charges passengers in economy seats for food and drinks on these flights. The airline’s managing director, Dominic Paul, said that it was responding to the demands of a “changing marketplace”.

BA has also cut luggage allowances, abolished free meals and begun charging for booking of particular seats on short-haul flights, as it attempts to recoup more than £400m in losses in the last financial year.

While many scheduled carriers report dwindling passenger numbers, low-cost carriers continue to see growth: easyJet reported a 9.3 per cent rise in passenger traffic in December compared with December 2008.

Analysts said the airline had benefited from the threat of Christmas strike action against BA and the disruption by snow of Eurostar services.

Ryanair announced this month that it carried more than 65 million passengers in 2009, an increase of more than seven million compared with the previous year, while Wizz Air carried 7.8 million, an increase of 33 per cent over 2008.

BA carried four per cent fewer passengers in December compared with the same month last year. Overall, passenger numbers fell by 750,000 to 25.2 million last year. It is not yet clear how damaging the renewed threat of strikes will be to bookings, but several travel agents have already switched flights away from BA since the cabin crew’s union, Unite, announced plans for a new strike ballot. The vote is expected to be held in early March.