(Bloomberg) – International airlines said European Union plans to make carriers comply with emission curbs constitute an illegal levy on carriers that will undermine efforts to fight climate change.

The International Air Transport Association told the EU Court of Justice in Luxembourg that carriers oppose on several grounds “unilateral action” that extends the EU carbon market to all flights that depart from or arrive at an airport in the region.

The EU plan “is not the right way to reduce international airline emissions, nor is it lawful,” said Conor Quigley, a lawyer for the International Air Transport Association, whose members account for about 93 percent of international flights.

United Continental Holdings Inc., AMR Corp.’s American Airlines and the Air Transport Association of America are leading the court challenge against the EU’s first attempt to extend the world’s largest carbon cap-and-trade program beyond its borders. The U.S. carriers claim the plan violates international law, the Kyoto Protocol, an EU-US Open Skies Agreement and the Convention on International Civil Aviation, the so-called Chicago Convention.

The measures are “a charge, levy or tax prohibited by the Chicago Convention and Open Skies Agreement,” Quigley said. They will “will seriously impede effective worldwide reduction of greenhouse gases.”

China’s airline association has said the measures, which start next year, are “unreasonable and illegal” and warned of a potential trade conflict. Willie Walsh, the head of British Airways parent International Consolidated Airlines Group SA, said last month “we fully expect” other states to retaliate.

The EU’s move, which follows a doubling of airline discharges in Europe over two decades, is a “practical example” of necessary action to prevent global warming, EU Climate Commissioner Connie Hedegaard said last month.

The Emissions Trading System, started in 2005, covers more than 11,000 utilities and manufacturers and is the cornerstone of the EU’s climate plan. It requires companies that exceed their carbon dioxide emissions quotas to pay a fine or buy spare permits from businesses that emit less.

Airlines would be the second-largest sector in the system, after power generators. Under the legislation, 82 percent of the emission allowances making up the airline-industry cap would be allocated for free and 15 percent would be auctioned. The remaining 3 percent would be put into a special reserve for later distribution to fast-growing airlines and new entrants.