ECTAA, the European association representing leisure and smaller business travel agencies, has warned the European Commission against full de-regulations of the GDSs.
It is the latest in a growing line of organisations and companies alarmed at the EC's apparent plans to repeal all regulations relating to the CRS code of conduct.
The Commission announced in October that as part of its driver to cut red tape it proposed to repeal, codify and modify 222 basic legislations and more than 1,400 related legal acts in the next three years.
GDS de-regulation, which has been on the agenda in Europe for two years, was included in the 1,400 legal acts and scheduled for early next year.
At its meeting in Athens last week, ECTAA members said this would completely de-regulate GDSs and "may bring great prejudice to travel agents and the traveling public."
ECTAA said that if mandatory participation by all GDSs was abandoned, there was a "high risk that parent carriers will only provide access to their fares through their own CRS which would lead to market dominance of the last remaining carrier-owned CRS on several major European markets.
"There is a great threat of abuse of dominant position by this CRS."
ECTAA is referring to Amadeus, a leading GDS partly owned by Lufthansa, Air France and Iberia. It is the only GDS in Europe in which airlines have a financial interest.
ECTAA also cited access to fares by agents as another issue raised by the possible full de-regulation.
"The CRSs are indispensable to the functioning of a neutral and independent distribution network by travel agents," it said.
The Association at its meeting also called for a revised code of conduct for CRSs to stop them selling agents' commercially sensitive information to airlines.
Carriers like BA, and Galileo and Sabre, rival GDSs to Amadeus, are among the leading companies protesting against the apparent EC plans for full de-regulation.
This would abandon the mandatory participation of airlines which have a stake in a GDS to display their contents on all GDSs in their home market.