The EU accused the WTO of "serious legal errors" in the case and said it might consider lodging an appeal.
The US trade representative slammed the ongoing subsidies as "massive EU corporate welfare" following the WTO's decision on Monday and insisted that American and European companies must compete on a "level playing field".
The reason is that the assistance that Airbus received in the early 2000s for the A380 programme have stopped harming the United States aerospace company due to the fact that the giant European plane is no longer being sold and production is set to come to an end because the aircraft has not sold as expected when it was conceived. It placed partial tariffs on most Airbus jets and products from cheese to olives and single-malt whisky.
Responding to Monday's ruling, the EU's executive commission faulted the panel for making a "number of serious legal errors in its assessment of European Union compliance", and said its recommended ways of compliance would be "very problematic for a larger part of the WTO membership".
The EU statement said: "We remain fully committed to working with the United States on a fair and balanced solution for our respective aircraft industries".
While the WTO no longer faulted Airbus for causing lost sales to Boeing with the A380, which is no longer marketed, it ruled that the superjumbo would cause market-share damage to Boeing for as long as it is produced and delivered.
Officials on both sides have expressed support for a negotiated settlement, while accusing the other of failing to take the prospect of a negotiated solution seriously.
Boeing shares were down 1.44% at $360.90 at the time of publication. Both planemakers have promised to comply with WTO rulings.
The US put tariffs on $7.5 billion worth of European Union goods in October, and US Trade Representative Robert Lighthizer said the new finding meant the US is considering increasing these tariffs rates and levying further tariffs on new goods.
US President Donald Trump said at a North Atlantic Treaty Organisation press conference on Tuesday: "We're going to tax Airbus... that's going to be very good for Boeing".
The US$7.5-billion tariff ceiling approved by the WTO was based on the amount of economic damage believed to have been caused to Boeing as a result of European support to Airbus, mainly in the form of government loans for the A380 and A350.
The USTR rejected that argument, saying it saw no basis in the report to justify reducing US retaliatory tariffs.
United States sources said only full compliance or a political settlement - which is widely seen as remote amid fragile economic ties - could legally cancel WTO approval for tariffs.
European executives warned against a "lose-lose" trade war.
A final resolution could be made more hard because the WTO's appellate body, which has final say in trade dispute cases, is set to become unable to hear new cases starting next week.
Any appeals launched after that date risk falling into a legal void, while it remains unclear whether the Appellate Body will be allowed to rule on appeals filed before then.