A group of United States airlines had argued that forcing them to participate in the potentially costly emissions-trading system infringed on national sovereignty and conflicted with existing international aviation treaties.

But in a final ruling, the European Court of Justice in Luxembourg affirmed an opinion issued in October by its advocate general, who had rejected that claim.

“The court confirms the validity of the directive that integrates aviation activities in the system for trading emissions quotas,” the ruling said. It added that the plan “infringes neither the principles of customary international law at issue, nor the Open Skies agreement” concluded with the United States in 2007.

The court’s decision comes amid increasing pressure from some of the biggest trading partners of the 27-member bloc to suspend or amend application of the legislation to expressly exclude non-European Union countries — at least initially. Failing that, several governments have vowed to take their own legal action or retaliate with countervailing trade measures.

Although airlines for now will receive most of the permits they will need free, the European Union estimates that ticket prices could rise by as much as 12 euros ($15.65) on some long-haul flights to cover the cost of the additional permits required.

Airlines for America, an industry lobby group and one of the plaintiffs in the case, said that its members would be required to pay more than $3.1 billion to the European Union from 2012 to 2020. It said its members would comply with the system under protest but would also review options for pursuing the case in Britain’s High Court, which had referred the original complaint to the European court in 2009.

“The court did not fully address legal issues raised and has established a damaging and questionable precedent,” the airline group said in a statement.

In a letter dated Dec. 16, Hillary Clinton, the United States secretary of state, and Ray LaHood, the secretary of transportation, said Washington would be “compelled to take appropriate action” if Brussels proceeded, though the letter did not specify what form that might take.

In a statement on Wednesday, Krishna R. Urs, the State Department’s deputy assistant secretary for transportation affairs, expressed Washington’s disappointment but did not broach the subject of retaliation.

“We continue to have strong legal and policy objections to the inclusion of flights by non-E.U. air carriers” in the system, Mr. Urs said. “We do not view the court’s decision as resolving these objections.”

He added: “The path the E.U. has chosen is hampering progress towards a multilateral solution that is more likely to bear fruit in terms of concrete progress in limiting greenhouse gas emissions from aviation.”

The court’s decision, and the pressure from Washington, leave American airlines in an uncertain legal limbo, particularly following a bill approved this year by the House that would bar them from participating in the European system.

A similar bill was introduced this month in the Senate but was not expected to be brought to a vote until after the new year.

China has also made known its displeasure with the European directive. This year it threatened to suspend purchases of jets made by the European manufacturer Airbus if Chinese airlines were included. A group of Chinese carriers has also threatened to bring a lawsuit, possibly in Germany, where the authorities will oversee the application of the system to several Chinese airlines.

Experts say the Chinese could argue that the European law violates the Kyoto climate agreement by requiring airlines from developing nations, which are exempt from emissions cuts under the treaty, to bear the same burdens as carriers from wealthier nations.

Algeria has already begun a case in France contesting the system, according to the Arab Air Carriers Organization, an industry group that includes the country’s main carrier, Air Algérie.

Exempting foreign airlines, however, would anger European carriers that would still be required to participate.

James Kanter contributed reporting from Brussels.

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