The WTO's report details findings first issued in private to the EU and U.S. in January.
It says the EU has demonstrated the U.S. gave Boeing "export subsidies that are prohibited" and recommends the U.S. either withdraw them or "take steps to remove the adverse affects."
The report is based on an EU complaint and the dispute will likely next go to a WTO appeals panel.
The Geneva-based trade body also has faulted European governments for illegally supporting local aircraft maker Airbus, a unit of European Aeronautic Defence and Space Co.
