The World Trade Organization has partly reversed its ruling last year that found the European Union had given aircraft maker Airbus illegal subsidies.

The decision by the WTO appeals panel upheld that European subsidies to Airbus had caused U.S. rival the Boeing Co. to lose market share in several markets. But not all of the funding received by Airbus can be considered illegal subsidies under international rules, the appeals board said Wednesday.

Both Airbus and rival the Boeing Co. claimed victory.

“It’s time for Boeing to accept this legal defeat and end the masquerade,” said Rainer Ohler, chief of communications for Airbus. “The WTO confirmation of the European loan system is a big victory for Europe.”

But Boeing said the WTO’s ruling confirmed that Airbus received $18 billion in illegal funds. Boeing officials said that Airbus and Europe have six months to comply with the WTO’s ruling.

“This is a clear, final win for fair trade that will level the playing field for America’s aerospace workers,” said Jim McNerney, Boeing chief executive, in a statement. “The WTO has concluded that launch aid and other illegal Airbus subsidies distorted the market, harmed U.S. industry and now must end.”

The appeals panel concluded that financial aid provided by France, Germany, Spain and Britain for the development of several Airbus planes “caused serious prejudice to the interests of the United States.”

But the panel overturned an earlier finding that certain grants, such as for research and development, had constituted a “prohibited export subsidy,” and said Boeing hadn’t lost market share in Brazil, Mexico, Singapore and Taiwan because of the subsidies.

The case is part of a long-running spat between Washington and Brussels over subsidies to aircraft makers.

Brussels has also hauled the U.S. before the global trade body over what it says are illegal subsidies to Boeing.

In March, the WTO ruled that Boeing received at least $5 billion in subsidies between 1989 and 2006 that were prohibited under international trade rules.

That ruling is also being appealed.