Ryanair is to appeal against an Italian fine of 3m euros (£2.5m) for failing to help passengers after cancelling their flights during the volcanic ash crisis.
Italy's civil aviation authority (Enac) said it knew of 178 cases of passengers who did not receive mandatory help, such as food, between 17 and 22 April.
Ryanair said it had not received official notification of any Enac fine.
It also said it had not been told of any violations or given a chance to defend itself during an investigation.
That, Ryanair said, made the fine unlawful.
Ash from Iceland's Eyjafjallajokull volcano triggered a massive shutdown of European airspace last month.
The ash cloud paralysed many airports and left an estimated eight million tourists and business travellers stranded.
Italy's air agency, Enac, accused Ryanair of failing to provide passengers at Rome's Ciampino airport with drinks, foods and accommodation as required by European law.
Enac found that most other airlines had managed to meet their obligations despite the difficult circumstances.
"At a time when all airlines across Europe were cancelling thousands of flights and disrupting millions of passengers, it is extraordinary that Enac could investigate and then impose a three million euro fine only upon Ryanair," said the airline's spokesman Stephen McNamara.
Ryanair had initially insisted it would only refund the cost of passengers' tickets, but later agreed to fully implement European regulations and pay all "reasonable expenses".
Under EU Regulation 261, 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.