Robert Zoellick, the US trade representative, fired a warning shot at the European Union on Sunday, saying it would be taking a big gamble if it linked the escalating dispute over subsidies to Boeing and Airbus to a long-running fight over US corporate taxation.

Mr Zoellick told the Financial Times that such linkage would be "a risky course. If one does that it puts pressure on others to do that. I hope that's not the course they choose."