Ryanair is planning to sue the European Commission, accusing it of not acting on claims the budget airline has made about rivals.
Ryanair has had plenty of recent dealings with the EU
The Irish firm alleges that a number of national carriers - including Alitalia, Lufthansa and Air France - are being propped up by their home governments.
Ryanair made the claims last year, but says there has been a "repeated failure" to investigate them.
Last month, the Commission blocked Ryanair's bid to take over Aer Lingus.
A fortnight ago, the firm launched a court appeal against that decision.
Ryanair said that this latest action, to be taken to the European Court, related to "hundreds of millions of pounds paid out in state aid to the European airline industry".
It alleges, among other things, that the French government has offered illegal subsidies to Air France's base airports, saving the airline about 1bn euros (£665m; US$1.35bn) over the past seven years
"Although Ryanair has called on the Commission several times to investigate these claims, the Commission has failed to do so," Ryanair said.
The airline said there was one rule for national carriers and state-owned airports and another for other airlines and regional bases.
When the Commission blocked Ryanair's Aer Lingus takeover, it said Ryanair's reputation for "aggressive retaliation" minimised the prospoect of competition between airlines, adding that the takeover could lead to raised fares.
Ryanair boss Michael O'Leary said Brussels was biased against his company.