The European Commission is being asked to tackle "dangerous developments" in airline distribution around discrimination against so-called neutral channels.
Regulators in Brussels received a letter this week from a large array of travel agency groups and lobbying organisations, such as the European Travel & Technology Services Association which has the GDSs and a number of online travel agencies as members.
In it, EC officials are warned that the recent decision by British Airways to alter its distribution strategy is a sign that large airlines are looking to "make competition a matter of designing clever, non-transparent distribution mechanisms, rather than competing on price and service".
BA parent group IAG is to introduce a surcharge later this year that will hit bookings that are not made through NDC-based connections.
This followed a similar programme first announced two years ago by Lufthansa to impose a fee on fares bought through GDSs.
The letter says:
"The more large airlines join this practice, the more difficult it will be to sustain an independent and neutral distribution channel where consumers can compare airlines objectively."
The Commission, the letter adds, has been told that such a situation would only get worse without intervention but for two years there has been no response.
Regulators have the "tools to defend transparency and neutrality in airline distribution", namely the existing EU Code of Conduct for airline computer reservation systems.
The letter concludes with a plea to enforce existing legislation, adding:
"Those rules were precisely adopted to avoid a situation where large carriers take distribution hostage to promote their own offers as opposed to those of their competitors to the detriment of competition and consumer choice."
Signatories to the letter include the GTMC, European Passengers Federation, GBTA and ECTAA.