Jacques Barrot, EC transport commissioner, warned that the EU could suspend any Open Skies agreement with the US if the States did not grant further concessions.
Mr Barrot said that these included access to the lucrative American domestic market by 2010.
Speaking to the European Parliament in Strasbourg, the commissioner said under the present proposed deal, the US had agreed to further liberalisation talks next year.
"Some people who don't like the agreement say we'll never get to this second stage because the US would have won out on the first stage. But that's not right.
"People are right when they call for a mechanism that will inevitably lead to a second stage and I've got that.
"We're going to start negotiations for phase two in January next year. If there's no second-stage agreement between now and mid-2010, we can suspend our side of the deal," he said.
Support for the proposed deal grew this week with Lufthansa, bmi and the UK business class only airline Silverjet adding their backing.
In support of its case, bmi published fiugres it said showed "how much the current protectionist Bermuda II agreement could be costing the UK traveller."
(The Bermuda II agreement is the one that restrcits the number of airleisn able to fly the London Heathrow-New York route to four: two from the US and two from the UK.)
bmi said the figures showed that "on average, UK travellers were paying 50% more for a business class ticket from Heathrow to the United States compared to the same class seat from other major European destinations."
Mr Nigel Turner, bmi's ceo, said: “The British Government needs to wake up and see how the current protectionist agreement is hurting UK travellers.
"It has knowingly supported this unlawful agreement and now is the time for it to act.
"The only beneficiaries to the current regime are the four airlines that keep a stranglehold on the transatlantic market where they are immune from any meaningful competition."
The strengthening of support leaves the UK government, BA and Virgin Atlantic as the only major opponents of the proposed deal.
The UK government has criticised the deal because it does not give EU airlines entrance to the US domestic market and because it does not change America's strict rules on foreign ownership of its airlines.
The proposed deal would give US airlines free access to all EU airports and scrap the Bermuda II agreement, allowing other carriers to fly the route.
EC transport ministers meet in Brussels on March 22 to consider the deal.
* see BTE's recruitment site www.businesstraveljobs.com