The EU executive Commission has sent letter to Ireland and several other member States warning against them to scrap bilateral airline pacts with the United States.
If the States do not comply within two months, the Commission can take them to the EU's top court and eventually levy fines.
The European Commission has now taken action against the 20 EU states which had signed such deals with Washington.
The Commission argues it has the sole right to conclude a pan-European airline deal with the US following a 2002 court judgment which condemned eight EU states for signing bilateral pacts with Washington.
It says the bilateral deals limits competition as they only guarantee national airlines traffic rights and therefore prevent carriers from offering the best deal on a transatlantic flight.
The Commission is using this legal precedent to force the other members of the 25-nation bloc with bilateral airline pacts with the US to rip them up.
The Commission sent warning letters to Spain, Ireland , Hungary, Poland, the Czech Republic, Slovakia and Malta on Wednesday, urging them to scrap the deals.
It also sent a second warning letter to France, Greece, Italy and Portugal.
The Commission has already decided to take the Netherlands to the European Court of Justice for failing to scrap its deal with Washington.
In 2002 the EU's top court condemned Austria, Belgium, Denmark, Finland, Germany, Luxembourg, Sweden and Britain for their bilateral airline deals with the US and the Commission has already launched legal action against these states for failing to comply with the EU court ruling.
EU Transport Commissioner Jacques Barrot will visit Washington on March 21st and 22nd in an effort relaunch stalled talks on an EU-US airline pact.
So-called "open skies" agreements generally allow carriers to fly more freely between countries, with fewer restrictions on access to national airports.