Ryanair could face substantial payouts after being found to have failed to care for passengers stranded by the Icelandic volcanic eruption.
Yves Bot, advocate general at the European Court of Justice, ruled that the budget airline was remiss in its customer care responsibilities following the Eyjafjallajökull eruption in 2010.
The ruling followed a claim by passenger Denise McDonagh, who was unable to fly to Dublin from Faro, on April 17 due to the ash cloud grounding flights.
She made a claim for €1,130 (£942) in damages to cover her accommodation, food and transport costs.
Ryanair claimed the disruption caused by the ash cloud was a disaster beyond the boundaries of EU regulations, which call for airlines to cover the cost of care during events classed as "extraordinary circumstances".
The district court in Dublin referred the case to court of justice, which ruled that the disruption was not outside the bounds of extraordinary circumstances.
"The provision of care is particularly important in the case of extraordinary circumstances which persist over a long time," the advocate general said.
"It is precisely institutions where the waiting period occasioned by the cancellation of a flight is particularly lengthy that it is necessary to ensure that an air passenger, whose flight has been cancelled, can have access to essential goods and services throughout that period.
"A limitation of the obligation to provide care would in some measure deprive the EU legislation of its effectiveness, since after a few days the air passengers concerned would be abandoned to their fate."
He added that it would not be disproportionate to impose an obligation on airlines to cover these costs as they are free to pass those costs on through price hikes.
"What is more, that is a policy which has already been put into effect by Ryanair, which introduced a special levy in April 2011 in order to cover the costs which it had incurred in providing care to passengers whose flights had been cancelled owing to the eruption of the Icelandic volcano."
The advocate general's findings must be confirmed by court, which tends to follow suit in the majority of cases.