A European commissioner has said "maximum pressure" would be put on airlines to pay passengers hit by the volcano ash disruption.
EU Transport Commissioner Siim Kallas said no airline should gain a competitive advantage by ignoring customers' rights to a refund or care.
He put the cost of the crisis to the aviation industry and tour operators at up to 2.5bn euros (£2.2bn).
He added that there might be a case for state aid to support airlines.
He described the ash, which cancelled 100,000 flights and left more than 10 million passengers unable to travel over a period of six days, as "exceptional".
Any payment of state aid to help airlines was down to agreement by member states, he added, but it must be done on a level playing field basis.
The volcanic eruption on 14 April triggered a shutdown of European airspace, starting with the UK, which acted on scientific advice from the Met Office in London.
The ban on flights in areas affected by volcanic ash was very costly for airlines, many of which now want compensation.
Speaking at a press conference in Brussels on Tuesday, Mr Kallas said that the cost of the shutdown to the aviation industry and tour operators was estimated to be between 1.5bn euros and 2.5bn euros.
He said the wider cost to the European economy was difficult to calculate so soon after the event.
But he said that there was not a "miracle pot of money" in the form of state aid that was sitting about waiting for such a crisis.
Mr Kallas said there needed to be pan-european enforcement of passengers' rights.
Under EU regulations, if a flight is cancelled, those flying with European carriers into or out of the EU have the right to a refund or to be re-routed.
If passengers chose the latter, they have the right to care - such as accommodation and meals - while they wait.
For some passengers, this means sending the receipts for hotels and meals into the airline for reimbursement.
Airlines believe this has left them with a large bill for an event that was beyond their control.
Ryanair and BMI initially said they would not cover all these expenses - but quickly reversed their decision.
Mr Kallas said he was not aware of any specific airlines that were flouting the rules.
EU transport ministers will meet on 4 May and Mr Kallas said that he would be bringing a package of measures to the table.
This included fast-tracking the idea of a single European regulator for the European skies, which would have enabled a more agile response to the crisis.
Other responses - which he said could be brought in immediately - included the possibility of a temporary halt to flight restrictions so more night flights could get stranded passengers home.
Route charges, which airlines pay to air traffic control, could also be delayed to help airlines cope with their immediate cash-flow problems.
"As we are getting back to normal, our focus can shift to relief measures for the industry," he said.
"This is about practical measures to provide relief to the air transport sector so they can weather this crisis."