GENEVA: The World Trade Organisation (WTO) ruled on Monday that a tax break from Washington state to help US planemaker Boeing develop its new 777X jetliner was a prohibited subsidy.

The WTO said the subsidy came in the form of a renewed cut in the state's main business tax for aerospace agreed in 2013, when Boeing was considering where to base assembly of the latest member of its long-haul jet family.

It rejected a complaint by the European Union against six further tax measures.

It is the third swathe of public support for Boeing or its European rival Airbus criticised by the WTO in a record transatlantic trade dispute dating back 12 years, and involving mutual accusations of tens of billions of dollars of aid.

The ruling, which can be appealed by either side, comes as the United States ponders potential sanctions against the EU over earlier WTO rulings against Airbus.