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 per cent 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 programme beyond its borders. Article continues below The US 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 on Tuesday. They \"will seriously impede effective worldwide reduction of greenhouse gases.\" Retaliation 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. Licence fee \"Allowances are essentially a licence permitting emissions and payment to the state constitutes a license fee,\" Quigley told the court. \"Even though allowances may be traded, no other property right arises. In so far as the member states sell allowances, this raises money for the benefit of the state. \"It is estimated that in excess of €1 billion ($1.45 billion) per annum will be levied in this way in 2012, rising to €4 billion is annually by 2020,\" Quigley said. Emissions from international aviation account for 2-3 per cent of global greenhouse gas discharges and their share is expected to rise in the coming decades as the industry grows, according to the EU. Region\'s main tool The EU system is the region\'s main tool to reach its target to reduce carbon dioxide by 20 per cent in 2020 compared with 1990 levels. The High Court in London referred the case to the EU Court of Justice in Luxembourg last year to clarify the legality of the emissions curbs. The ATA has said it was \"compelled\" to file its lawsuit in the UK because the country was the first to implement the EU law.