Budget airline Ryanair has confirmed it plans legal action against other airlines if a European Commission subsidies decision goes against it.
Michael O'Leary says a European ruling could halt air fare subsidies
Ryanair expects regulators to rule on Tuesday that discounts it got from Belgian authorities to use Brussels Charleroi airport were illegal.
But Ryanair warns it will launch legal action against all airlines that use state airports offering concessions.
The carrier could be forced to pay back millions of pounds over the case.
The outcome could have ramifications for many Britons who have bought property overseas on the basis of having access to cheap air travel offered by low-cost carriers.
Nearly all European airports that operate at less than capacity offer discounts to encourage airlines to fly to them.
The Dublin-based carrier says a negative ruling would mean Belgian and European consumers would pay higher fares or may even have services discontinued as a result.
Ryanair said in a statement it will appeal against any unacceptable decision.
But it also warned: "Ryanair... has instructed its advisors to initiate state aid cases and complaints against every other airline flying into every state airport which offers concessions and discounts."
Chief executive Michael O'Leary also voiced his concerns on the issue on the BBC's Breakfast with Frost programme.
He said: "The real concern in this decision is they (European regulators) are attempting to enforce these guidelines on all other airlines in Europe.
"This means, effectively regional airports, that are publicly owned... now will no longer be allowed to offer discounts to airlines in return for low fare air travel."
Ryanair's tussle with the EU comes after the airline was successfully sued by passenger Bob Ross last week after it charged him £18 for the use of a wheelchair at London Stansted airport.