EU regulators are investigating whether three regional German airports and one in Austria may have breached EU state aid rules in setting lower charges for budget airlines Ryanair, Lufthansa's Germanwings and Air Berlin.
The European Commission said in a statement it would also look into capital injections and operating aid granted by German authorities to the airports at Saarbruecken, Zweibruecken, Luebeck-Blankensee and those provided by Austrian authorities to Klagenfurt airport.
The investigation, launched on Wednesday, is the latest move by the European Union executive against airports suspected of receiving unfair support. The Commission is also investigating possible illegal aid for airports in France, Sweden, Ireland, Hungary and Romania.
"On the basis of the information at its disposal the Commission cannot exclude that the measures in favour of the four airports and their customer airlines involve state aid which gives them an unfair advantage vis-à-vis their competitors and is thus incompatible with the internal market," the statement said.
It cited concerns with discounted user charges offered by Saarbruecken airport to Cirrus Airlines and Air Berlin, Zweibruecken airport's lower charges for Germanwings, TUIfly and Ryanair, and Luebeck-Blankensee airport's deals with Ryanair.
It also pointed to Klagenfurt airport's discounted scheme for Ryanair, TUIfly and Air Berlin.
The Commission can order national authorities to recover aid if this is found to have infringed EU state aid rules.