Some airlines have been challenged over their financial treatment of passengers affected by the volcano ash disruption.
The Irish Commission for Aviation Regulation said that Ryanair was breaking EU regulations by limiting expenses payments for food and hotels.
Holiday group Tui also said that it would charge passengers the difference if they ended up on more expensive flights than originally booked.
Thousands of travellers have faced lengthy waits because of the ash cloud.
Under EU regulations, if the original flight was cancelled those flying with European carriers into or out of the EU have the right to a refund or being re-routed.
If passengers chose the latter, they have the right to care - such as accommodation and meals - while they wait.
Ryanair boss Michael O'Leary described the care rules as ludicrous. He said the airline would consider all passenger requests for "reasonable receipted expenses" but the reimbursement would be limited to the original air fare paid by each passenger.
"There is no legislation designed that says any airline getting a fare of 30 euros (£26) should be reimbursing passengers many thousands of euro for hotel accommodation. It is absurd," he said.
But the Irish Commission for Aviation Regulation has taken up the case with the airline.
"While the Commission appreciates that the financial implications for airlines of the recent events are significant, it wishes to point out that the position adopted by Ryanair is incompatible with [the regulations]," the Commission said.
"The rights of passengers in respect of expenses incurred in relation to care and assistance measures are required to be fulfilled by the air carriers notwithstanding the circumstances of the current difficulties affecting the aviation sector.
"The Commission therefore reminds passengers to retain all relevant receipts in relation to necessary expenses incurred in respect of food and accommodation while awaiting new flight arrangements by their air carrier."
And Ryanair's stance was criticised by Rochelle Turner of consumer publication Which? Holiday.
"The regulations are very clear in what they are allowing consumers to claim for," she told the BBC.
"You can't just turnaround and say we can only pay the amount that your ticket was bought for. You just can't do that. Its not in the regulations. It's against the law.
The disruption has led to differing interpretations in the law on new flights.
Some flight-only travellers with Tui - which owns Thomson - whose original flights were cancelled are being told that they would have to pay the difference if an alternative flight the passenger chooses is more expensive.
"Tui UK and Ireland can confirm that customers who rebook to a different flight of their choice which is more expensive than their original must cover the difference themselves. Similarly, if the flight is cheaper Thomson Airways will refund the difference," the company told the BBC.
But the official airline watchdog in the UK, the Air Transport Users Council, disagrees.
"If the airline tells you that your flight is cancelled and you want a new flight rather than a refund, the date of the new flight should be at your convenience - not subject to any time limit - and you should not have to pay any more money," it said.
And Jacqueline Beadle, a solicitor at Gullands in Kent, said there was nothing in the EU regulations that said people have to make up the extra cost.
However, those who have chosen a refund and rebooked themselves would have to pay the current price quoted.
Those who did not wait to be re-routed and instead made their own travel arrangements to get home have to make a claim for that cost to their airline. If the cost is deemed unreasonable, they might not get all their money back. Equally, they might not get refunded for accommodation and food costs.