A programme to introduce a Single European Sky to cut flight delays has suffered a setback in a row over costs.
Planners hope to harmonise Europe's airspace to give aircraft speedier routing according to traffic flows, rather than national borders.
Work on this has already started, but the umbrella body CANSO, the Civil Air Navigation Services Organisation, said this week it had "strong concerns" about savings targets of 4.5% suggested for 2010-2014.
Graham Lake, CANSO director general, warned: "The adoption of unrealistic targets risks damaging the credibility of the performance scheme."
But Giovanni Bisignani, chief executive of the International Air Transport Association, said the industry needed to "abandon their outdated mentality that is crippling air traffic management."
He added: "Europe's air traffic management is a mess and it needs to get better.
"The need for Europe to achieve the efficiences of the Single European Sky was evident for the whole world during April's volcanic ash shutdown. And passengers suffer daily from air traffic control delays or circuitous flight routings."
If implemented by 2020, the current target date, the Single European Sky should bring cost benefits of €5 billion a year.
A CANSO spokesman said it accepted targets were needed, but these "had to be on a realistic basis". CANSO said targets were unrealistic because they were based on rates set before 2009, when air travel boomed. He said the system became less efficient when fewer aircraft were in the sky.
"You can't reduce the fixed infrastructure of the radars and the people managing the aircraft."
CANSO also warned performance targets for environmental efficiency, an improvement of 10% by 2014, are also unrealistic, as they are based on similar projections.
The spokesman added: "In the last couple of years there has been a decline in traffic, so it has become easier to route aircraft more directly, which means they burn less fuel."
As traffic returns, it becomes harder to do this. "If you have lots of aircraft in the sky, you might have to put them in a stack, or take them on a wider angle in order to ensure that everyone is moving around the air safely."