Boeing 777X
Boeing 777X (pictured) is meant to compete with rival Airbus' A350, with both sides involved in bitter battles at the WTO. Boeing

The World Trade Organization's appellate body handed a key victory to Boeing on Monday (4 September) by rejecting European Union claims that tax incentives provided to the US aircraft maker by Washington State were illegal and constituted state aid.

The long-running dispute saw Boeing's European rival Airbus win a claim in 2016, backed by the EU, that around $9bn (£7bn) in aid was provided to the US aircraft maker to help build its 777X model.

But Boeing and US government appealed the decision last year and secured an outright victory that is not subject to further appeal, according to the Geneva-based WTO.

The latest ruling now goes to the WTO's dispute settlement body for formal adoption within 20 days.

Boeing has always maintained that the tax breaks amounted to no more than $1bn. Furthermore the aircraft maker said that the breaks it received did not constitute state aid.

The Boeing 777X is currently being billed by aviation experts as the world's largest and most efficient twin-engine jet, with higher cabin ceilings, new lighting systems, and capabilities of providing more natural levels of humidity and passenger comfort.

It is meant to compete with Airbus' A350 model.

Responding to the decision, the US aircraft maker said: "The WTO has rejected yet another of the baseless claims the European Union has made."

However, Airbus said ‎it would continue the fight as other complaints over alleged US aid to Boeing are yet to be resolved.

Two larger cases, involving on multiple claims and counter-claims by both sides about alleged illegal subsidies, for their respective aviation industries, remain pending before the WTO.

Boeing 777X cabin
The cabin of the Boeing 777X will have higher ceilings and improved levels of passenger comfort.