The European Union (EU) has accused the US of giving Boeing almost $24bn (£12.2bn) in state aid.
Boeing has been supported with US aid of $23.7bn (£12.2bn)
In the latest round of a long row over aircraft subsidies, the EU said this had hurt sales of Airbus planes.
The claim came in written evidence to a World Trade Organisation (WTO) panel probing the EU's complaint against Washington DC.
The transatlantic dispute over illegal aircraft assistance is the biggest to come before the global arbiter.
The EU and US have been at loggerheads for years, exchanging charges and counter-charges that each provides illegal state aid to its main aircraft maker, Boeing in the United States and Airbus in Europe, in breach of WTO fair-trading principles.
The EU has estimated the US administration had given Boeing $18.9bn in the form of aid for research and development and tax relief from 1990 to 2004 and that individual states had provided $4.8bn.
"The EU's submission exposes in detail the massive, long-standing and WTO-inconsistent subsidisation of Boeing's civil aircraft division," the 27-state bloc said in a statement."
Boeing rejected the allegation, arguing some of the programmes Europe mentioned were not related to civil aircraft while others were available to a range of companies, including its arch-rival Airbus.
"They are not subsidies," said Boeing spokesman Charlie Miller. "In any event, even if all of the European Commission allegations had merit - which they don't - they are dwarfed by the magnitude of the subsidies that Airbus has benefited from."
The US has accused Britain, France, Germany and Spain of supporting Airbus' development to the tune of $15bn - backing which it argued had saved Airbus as much as $100bn over the years.
"Launch aid has enabled Airbus to launch a series of large commercial aircraft models at a scale and a pace that would have been impossible without subsidies," a US official said. The US says it intends to seek the immediate return of some $4.5bn if it wins.
But EU representatives dismissed the $100bn figure as "a smoke screen" and argued that state aid to Airbus, unlike the assistance provided Boeing, must be reimbursed.
The WTO is set to issue a preliminary ruling on the US complaint against Airbus in September while the EU suit targeting Boeing is to get a hearing in July, with a decision unlikely before February 2008.