“The WTO will decide what the proper number is and .. give the U.S. that authority,” Robert Novick, co-managing partner at U.S. law firm WilmerHale, told the BBC Today programme.
“In parallel, the U.S. will develop a list of products on which it might consider imposing counter-measures,” he added.
The transatlantic dispute stems from mutual claims that the world’s two largest planemakers benefited from illegal aid in the form of subsidized government loans to Airbus and research grants or tax breaks to Boeing.
Underscoring the cost and complexity of the case, the two sides have been arguing since 2011 about whether they complied with earlier rulings.
Airbus did not say how it would comply with the final ruling on European aid but a European Commission document said it would repay an A350 loan to the UK government this year and reduce the drawdown of other loans.
It also said the bankruptcy of Russian carrier Transaero, resulting in fewer A380 deliveries, had helped it to comply, while other aid had been blunted by the passage of time – an argument that has previously been rejected by Washington.
Karl Hennessee, senior vice president and head of litigation at Airbus, told BBC Today that Airbus wanted a peace settlement similar to one between Canada and Brazil that set the tone for global aircraft export financing.
Nevertheless, Boeing has appeared to rebuff the offer.
“The most important message that Europe and Airbus can send to the rest of the world about the rules of trade in civil aircraft is to comply with this decision,” Novick told the BBC.