Airlines are deliberately flying longer routes over the Atlantic Ocean to avoid paying air traffic control charges, a BBC Investigation has discovered.
Tango routes incur fewer aircraft control charges
Thomas Cook and Monarch frequently burn extra aircraft fuel to avoid higher fees on flights to the Canary Islands.
The 100-mile (160km) diversion, known as the tango route, can produce an extra three tonnes of carbon dioxide.
Both airlines said they used the routes to avoid air traffic congestion and maintain schedules.
During a month-long investigation the BBC South East team monitored 44 Thomas Cook flights from Manchester, Newcastle and Birmingham flying to the Canary Islands and back.
The direct route, flying over French, Spanish and Portuguese airspace, incurs £1,578 in air traffic control charges.
Tango route costs come to £968 which, allowing for the extra fuel costs, means the airlines would make an average saving of £99 per flight.
The BBC South East Investigations' team was tipped off about tango routes by a Thomas Cook pilot.
The pilot, who has asked for anonymity, said: "There are parts of Europe which are cheaper to fly over than other parts.
"So you can lengthen a flight by maybe fifteen minutes or more to avoid expensive bits of airspace.
"To get into the tango routes you have to fly out over Ireland."
The BBC team showed the 44 flight plans to a former airline operations director.
The director, who wanted to remain anonymous, said: "The overall trend is that there is a high proportion of flights taking ocean routes when there was no clear wind advantage to do so.
The CO2 produced was equal to 150 car journeys to Brighton
"I would hazard a guess that overflight charges may have been factored into these routings, owing to the disproportionately higher cost of the direct routing. I can understand why this might be done.
"The tango routes are quieter, and are less likely to suffer slot restrictions. Airlines may be able to benefit from more punctual schedules. Schedule punctuality is really important for the charters."
Andy Farrar, of Air Data, based in Gatwick, calculated the fuel burnt on a tango route flight from Manchester to Tenerife on 16 November and compared it with an imaginary direct flight.
Mr Farrar said: "The flight which flew over the ocean used 14.7 tonnes of fuel and took four hours 17 minutes.
"The direct route would have used 13.1 tonnes and have been shorter at three hours 57 minutes."
Mari Martiskainen, a climate expert at the University of Sussex, said the extra fuel would produce three additional tonnes of CO2 per flight.
She added: "That's equal to the amount of CO2 emitted by 150 car journeys between London and Brighton."
Thomas Cook's environmental policy is to 'continuously strive to improve our environmental performance and to minimise any negative impact resulting from our operations'.
The German-owned company said: "Thomas Cook Airlines can confirm that it does operate routes to the Canary Islands, which include using tango routes.
"These routes are used when they are the most efficient and when it is necessary to avoid lengthy air traffic control delays caused by airspace congestion in European airspace.
"On the Thomas Cook Airlines flights that used tango routes between the 2nd and 16th November 2007, fuel was saved on more than 75 per cent of these flights."
Luton-based airline Monarch admitted using the tango routes to avoid paying overflight charges but it also said it used them to maintain flight schedules.
A spokesman said: "By travelling via these Oceanic routes, the company avoids paying French and/or Spanish overflight charges and instead pays a much smaller Oceanic airspace overflight charge to the UK and Ireland.
"However, this overflight cost saving has to be balanced against the additional mileage which increases both the flight time and fuel-related costs.
"In a highly competitive marketplace, where customers demand increasingly low fares, we have to manage our costs as tightly as possible.
"Ultimately the environmental impact comes down to whether the travelling public is prepared to pay."