A British Airways whistleblower has revealed an industry-wide practice that deliberately adds weight to flights, increasing greenhouse gas emissions.
"Fuel tankering" sees planes filled with extra fuel, usually to avoid paying higher prices for refuelling at their destination airports.
It could mean extra annual emissions equivalent to that of a large town.
BA said it was common to carry extra fuel for "operational, safety and price reasons".
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 flights involve 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 this was a "classic example of a company putting profit before planet".
He added: "This is why we can't afford another decade of believing corporate greenwash and waiting for the voluntary carbon reductions to appear.
"We need tough regulations to limit aviation's emissions, because so long as there's money in polluting, they'll pollute as much as they can."
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.