The Spanish Consumer Association has accused Ryanair of jeopardising passengers' safety after three of its planes made low-fuel emergency landings on the same day last month.
The Irish Aviation Authority and its Spanish equivalent are investigating the mayday calls made at Valencia Airport in Spain on July 26.
Bad weather resulted in the flights being diverted from Madrid and, after being in a holding pattern for almost an hour, the pilots issued the mayday calls and asked for permission to land immediately.
Evan Cullen of the Irish Airline Pilots' Association claimed Ryanair's "corporate culture" was responsible for the three pilots issuing the mayday calls.
He claimed Ryanair's league tables on the use of fuel led to the three planes making the low-fuel emergency landings.
Mr Cullen said Ryanair stipulates the amount of fuel that each flight should have and that pilots are required to keep refuelling to a minimum.
"In my 23 years as a pilot I never made a mayday call, most pilots would expect to get through their entire career without ever having to make a mayday call.
"The only time you would use this is in the simulator during your training. We are saying a corporate culture is driving pilots to do things they are not comfortable with," he said.
However, Ryanair spokesman Stephen McNamara has denied its fuel policy was responsible for the mayday calls.
He said the mayday calls occurred in extraordinary circumstances after more than 70 minutes of extra flight time.
"This was an extremely unusual situation. These aircraft, which had already flown for three hours to get to Madrid, found that Madrid could not let them in, so they diverted to Valencia.
"They already had 70 minutes of extra flying when they realised they had to, as per the regulation, land. They know they have to land with 30 minutes of contingency fuel remaining," Mr McNamara said.
The Spanish consumer association has filed a complaint against Ryanair with the civil-aviation authority on the grounds the airline was jeopardising passenger safety.
The association asked for the airline's operating licence to be suspended for three years and for a fine of ?4.5m to be imposed
"Aviation is the most highly regulated industry in Europe. Every airline works within the parameters put out there. We are never going to put fuel before safety," Mr McNamera said.