The European Commission is to probe an annual payment of 400,000 euros (£274,000) by France's publicly-owned Pau airport to budget airline Ryanair.
Is Ryanair flying into more trouble?
A Commission official told BBC News Online that a complaint had been received regarding the deal and it was duty-bound to hold an investigation.
The Commission ruled on Tuesday that Ryanair received illegal incentives to use Belgium's Charleroi airport.
It said Ryanair would have to repay some of the money it saved.
An investigation of its Pau route, which serves southern France, would be a further blow to the company which suspended flights to Strasbourg in 2003 after a French court backed an Air France complaint.
A subsidiary of Air France said that Strasbourg airport's deal with the Irish carrier was illegal state aid.
Some analysts have suggested that Ryanair agreements at Montpellier in France, Malaga in Spain and Pisa and Venice in Italy are also possible candidates for a complaint.
A Ryanair spokeswoman said the firm was aware a complaint regarding the Pau airport deal had been made.
Budget airlines have been watching the Commission's ruling on Ryanair's Charleroi operation closely.
It has been said the spotlight could fall on other contracts carriers have with airports in Europe, many of which are publicly-owned.
Easyjet, for example, has an arrangement at Berlin's publicly-owned Schoenefeld airport, and agreements in Marseilles and Toulouse.
Germanwings flies to Cologne/Bonn airport while Bmibaby has a contract with Manchester, which is owned by the local authorities.
But the airlines say the deals they have with the airports are not the same as those Ryanair negotiated.
"Ryanair are trying to suggest that we have similar agreements with publicly-owned airports in order to make it [the ruling] an industry issue and win sympathy for their case," an Easyjet spokesman said.
"But the agreements we have are not the same."
Ryanair is accused of trying to make the ruling an industry issue
Germanwings said its business plan was very different to Ryanair's.
"We pay the published fees and don't get subsidies from the state... our business plan is not based on public subsidies," said a spokesman, Joachim Schoettes.
Bmibaby admitted it had negotiated favourable landing fees with public airports but that the deals were available to its competitors.
"It is our understanding that the arrangements that are in place at airports served by bmi are either within published tariff agreements or are within the guidelines set out by the European Commission," said a spokesman.
The Commission's transport office said there were no complaints against other airlines at present.
Michael O'Leary, the Ryanair chief executive, has described the ruling on the perks it received to use Charleroi as a "disaster".
But industry analysts are wondering why more of its deals have not been scrutinised.
"We need more transparency. What is happening to public money? How much and who to? That's a reasonable demand," said David Learmount of Flight International magazine.
But Ryanair was unlikely to leave Charleroi, he predicted.
"One might notice when a ruling went against him in a French court over the Strasbourg route he said 'I'm out of here' and left straight away," Mr Learmount said.
"Have you heard him say he's going to go? He's annoyed but he's not going to pull out... he's making money."
Ryanair has pointed out that jobs in Charleroi would be lost if it decided to leave because of the ruling.
Attracting airlines to regional airports can rejuvenate the sometimes depressed areas around them.
The president of Pau's Chamber of Commerce and Industry (CCI) Michel Brau told the AFP news agency that the deal its airport has with Ryanair results in the arrival of 50,000 British passengers in Pau every year.