Europe's busiest airports could carry 28 million more passengers every year without further expansion or extra runways, a report claimed today.
The study, ordered by the European Commission (EC), says that more efficient use of take-off and landing slots could absorb some of the current congestion.
Demand already exceeds capacity most or all of the day at six European airports - Heathrow, Gatwick, Paris Orly, Milan Linate, Dusseldorf and Frankfurt.
While some major European airports are planning to expand and may be able to cope with the increase, others, including Heathrow, do not plan expansion and face worsening "capacity constraints", the report warns.
There are currently 26,000 flights using Europe's airspace every day, with estimated growth of 5% a year.
Better planning and use of slots alone could generate more than five billion euro (£4.3 billion) in economic benefits by 2025, the report estimates - adding up to 28 million passengers without any other modifications to airport capacity.
EU Transport Commissioner Siim Kallas said: "We have been concerned that the current system of allocating take-off and landing slots at airports is inefficient, giving rise to delays and congestion.
"This has now been confirmed by today's report, showing that up to 28 million more passengers could travel each year through Europe's airports."
The commissioner said he intended to propose legislation later this year to tackle the issue.
The study, drawn up by an independent transport planning consultancy, says problems with the current slot allocation arrangements at Europe's airports not only causes congestion but hinders competition between airlines.
The system of slot co-ordination cannot generate more airport capacity, the report points out, but the way slot allocation is organised should ensure that limited capacity is used as effectively as possible.
"At some airports this does not occur because of factors which include a significant proportion of small aircraft, limiting the number of passengers that can be transported within the constrained capacity," the report says.
Despite significant new competition in the European air transport market, including the growth of low-cost airlines such as easyJet and Ryanair, the system of "historical preference" makes it very difficult for new entrants to challenge the dominant position of traditional airlines at the most congested airports.
The turnover of slots remains very low, with established carriers having little incentive to give them up, "even when other carriers could use them more effectively than they could".
An EC response to the report said the study had highlighted the need for more transparency in the way airline slots are used at airports, including a stricter "minimum-usage" requirement to limit established carriers' ability to retain slots they are not using even when there is demand for them from other carriers.
Mr Kallas said he would now be proposing legislation later this year to encourage the most effective use of slots.