British Airways has launched a review into a money-saving practice which increases its greenhouse gas emissions.
It follows a BBC investigation exposing "fuel tankering" by airlines - in which planes are filled with extra fuel, usually to avoid paying higher prices for refuelling at destination airports.
The industry-wide practice could mean extra annual emissions equivalent to those of a large European town.
BA now says that using tankering to cut costs "may be the wrong thing to do".
However, the airline added that it also uses the practice for safety and operational reasons, including helping planes to turn around quickly.
BBC Panorama has discovered the airline's planes generated an extra 18,000 tonnes of carbon dioxide last year through fuel tankering.
Cost savings made on a single flight can be as small as just over £10 - though savings can run to hundreds of pounds.
Researchers have estimated that one in five of all European airlines' flights involves some element of fuel tankering.
The practice on European routes could result in additional annual greenhouse gas emissions equivalent to that produced by a town of 100,000 people.
Critics say the widespread use of the practice undermines the aviation industry's claims that it is committed to reducing its carbon emissions.
John Sauven, Greenpeace UK's executive director, told the BBC that fuel tankering was a "classic example of a company putting profit before planet".
Responding to BA's decision to carry out a review, Mr Sauven, said it showed how the airline industry had been treating climate change "like a PR problem".
"This is why we need government-enforced reduction targets to ensure airlines take responsibility for the damage their emissions are causing," he said.
International Airlines Group (IAG), the company that owns BA, says it wants to be the world's leading airline group on sustainability.
BA boasts it even prints its in-flight magazine on lighter paper to save weight.
Yet BBC Panorama has seen dozens of internal BA documents that show up to six tonnes of extra fuel have been loaded onto planes in this way. It has also seen evidence that Easyjet carries extra fuel in this way.
Airlines can save money from the fact that the price of aviation fuel differs between European destinations.
Make a profit
BA insiders say the company - like many airlines running short haul routes in Europe - has computer software that calculates whether costs can be saved by fuel tankering.
The software will calculate whether there is a cost saving to be made. If there is, crews load up the extra fuel.
An example of documents seen by Panorama show that a recent BA flight to Italy carried nearly three tonnes of extra fuel.
The extra weight meant the plane emitted more than 600kg of additional carbon dioxide - the same emissions one person is responsible for on a return flight to New York.
The cost saving on that trip was less than £40, but the documents Panorama has seen show that it can be even lower than that.
IAG made an annual profit of €2.9bn (£2.6bn) in 2018, about 80% of which came from BA.
A BA insider described the practice as "hypocritical".
"For such a big company to be trying to save such small amounts while emitting so much extra CO2 seems unjustifiable in the current climate," he said.
In response to the claims, the chief executive of BA's parent company, IAG, announced the airline would carry out a review of the practice.
On Friday, Willie Walsh told investors that the airline wanted to ensure it was not "incentivising the wrong behaviour" from managers.
"Because clearly the financial saving would have incentivised us to do fuel tankering," he said.
"But maybe... this the wrong thing to do and the wrong thing to incentive. So we want to make sure we have our incentives aligned to the right activities so ensure financial sustainability but also environmental sustainability."
BA said it was common practice for the airline industry to carry additional fuel on some flights.
The airline said for BA this applies mainly to short-haul destinations "where there are considerable price differences between European airports".
It said the additional emissions from the airline represented approximately 2% of the total extra emissions generated by all airlines tankering fuel in Europe, based on research by Eurocontrol.
BA pointed out that since 2012 all flights within Europe had been covered by the EU Emissions Trading System.
It added that from 2020 the company would offset all CO2 emissions from its UK domestic flights.
Easyjet said it had reduced the level of tankering in recent years and that it only took place on a tiny proportion of flights for operational and commercial reasons.
Eurocontrol, the body which coordinates air traffic control for Europe, has calculated that tankering in Europe resulted in 286,000 tonnes of extra fuel being burnt every year, and the emission of an additional 901,000 tonnes of carbon dioxide.
It calculates that the practice saved airlines a total of €265m (£228m) a year.
Eurocontrol described the practice as "questionable" at a time when aviation is being challenged for its contribution to climate change.
Panorama: Can Flying Go Green? is on BBC1 at 20:30 GMT on 11 November.