The WTO on Wednesday dealt the European Union a painful blow in a transatlantic trade row over multibillion dollar subsidies for US and European aircraft, ruling some state support for Airbus illegal.

Rival US airplane manufacturer Boeing claimed a "sweeping legal victory" and said it would require Airbus to repay four billion dollars in illegal subsidies, a claim disputed by the European aerospace giant.

Bringing to a head one of the most bitter trade disputes between the two trading powers, the World Trade Organization disputes panel upheld parts of a US complaint in the marathon legal battle.

In a 1,200-page ruling made public for the first time, the global trade referee said EU states should halt some aid for the development and export of Airbus airliners.

It notably accepted three out of seven claims by Washington that key launch aid amounted to export subsidies, which are illegal under WTO rules.