The World Trade Organization ruled on Monday that a tax break from Washington State to help Boeing develop its new 777X jetliner was a prohibited subsidy. The ruling was a setback for the aerospace giant as it seeks victory in a parallel case against its European rival, Airbus.

The trade organization said the subsidy came in the form of a renewed cut in Washington State’s main business tax for aerospace. The state agreed to the tax cut in 2013, when Boeing was considering where to assemble the latest member of its long-haul jet family.

It is the third time that taxpayer support for Boeing or Airbus has been faulted by the World Trade Organization in a trans-Atlantic trade spat dating back 12 years and involving mutual accusations of tens of billions of dollars of aid.

The ruling can be appealed by either side. The United States is considering sanctions against the European Union over Airbus subsides that were previously ruled illegal.