Two Star Alliance codeshare relationships are to be investigated by European competition regulators, to assess whether they are in line with antitrust laws.
The European Commission is to examine the arrangements between Lufthansa and Turkish Airlines as well as a similar pact between Brussels Airlines and TAP Portugal.
Inquiries are to concentrate on the airlines' agreements to cross-sell tickets on three trunk routes: Frankfurt-Istanbul, Munich-Istanbul and Brussels-Lisbon.
The Commission says that, on these routes, the carriers "should, in principle, be competing with each other".
"This form of free-flow, parallel, hub-to-hub codeshare agreement may distort competition leading to higher prices and less service quality for customers," it adds.
It points out that the two investigations are separate, and the Commission has not given a deadline for completion.