The two companies disagreed over the extent of the subsidies outlined in the report, which was delivered to the U.S. government and the EU Commission but not released publicly.
Airbus said the report showed Boeing had received at least $5 billion in illegal subsidies and was only able to launch its 787 Dreamliner with such support. Boeing denied the assertions.
The United States and European Union, both trading superpowers, have been fighting cases against each other in the WTO for more than six years over each other's subsidies for large passenger aircraft.
Boeing's stock, a Dow component, showed little reaction to the WTO report. Shares were up just 8 cents at $69.31.
"I don't think there's clarity as to what this really means. It seems to be a lot of noise," said Alex Hamilton, managing director of EarlyBirdCapital, a boutique investment bank.the dispute as Airbus and Boeing battle for the $1.7 trillion market.
The report is still confidential, but even Boeing acknowledges that the WTO has backed some of the EU claims. However, the two sides disagreed strongly over the amount of condemned Boeing subsidies and how they compared with those given to Airbus.
The findings came as a U.S. Air Force decision was expected within weeks on whether to award Boeing or Airbus parent EADS a contract worth $25 billion to $50 billion for refueling tankers. But EarlyBird's Hamilton said the WTO report was likely to have little impact on the U.S. government decision.
Both plane makers have supporters in the U.S. Congress, as Boeing would build and finish the tanker in Washington state and Kansas while Airbus would assemble it in Alabama.
Senator Richard Shelby of Alabama said that the WTO report "unquestionably states that Boeing received significant government subsidies prohibited by the WTO."
"Today's decision should end Boeing supporters' attempt to derail the tanker competition by arguing that the trade dispute is one-sided," Shelby said in a statement.
On the other side, Senator Pat Roberts of Kansas said: "Attempts to skew this ruling in the EU's favor are an exercise in distraction. Once this ruling is made public in the near future, the sunshine will refute much of the claims by the EU and truly indicate where the market-distorting benefits flowed."
With no end to the litigation in sight, both sides have periodically called for a negotiated end to the dispute, as EU Trade Commissioner Karel De Gucht did in September when an interim version of the report was issued..
His spokesman said on Monday the report confirmed those initial findings, in comments suggesting Brussels was not keen to raise the stakes, or wanted to play down a previous WTO ruling that condemned its own support for Airbus.
"This solid report sheds further light on the negative consequences for the EU industry of these U.S. subsidies and provides a timely element of balance in this long-running dispute," spokesman John Clancy said in a statement.
One EU diplomat said a political solution was preferable
"Given the state of the global airline industry today, a political agreement is important for both sides of the Atlantic," the diplomat said.
The ruling, like the interim report, was handed only to the parties. It will not be published for several weeks while being translated into French and Spanish.
U.S. Trade Representative Ron Kirk's office said it could not provide detailed comment now as the report was confidential.
"Despite that the EU has publicly commented on the report, at this time we will simply say that the United States is confident that the WTO will confirm the U.S. view that European subsidies to Airbus dwarf any subsidies that the United States provided to Boeing," spokeswoman Nefeterius McPherson said.
Massive Subsidies
Airbus said the report would show Boeing had received billions of dollars
