United Airlines has sent a memo to select travel agents, revoking their authority to charge credit cards using the carrier’s credit card merchant facilities, effective July 20. United has not indicated specifically to which agents the notice was sent, however some travel agents in the blogosphere have indicated they believe the carrier sent the memo to agencies with limited United revenue generation, to minimize the effects should the plan meet negative reaction.

Many have called the move similar to one by Northwest in 2004 where the airline attempted to begin charging travel agents to sell tickets via the Global Distribution Systems (GDS) used by most agents. That move was negated after several GDS operators intervened, saying it was a violation of Northwest’s user agreement to tack on fees for issuing tickets via their systems.

United’s move affects agents in a similar way, but with greater ramifications for agents. While Northwest’s attempt would have increased the cost to issue tickets for a travel agent alone, United’s proposal would effectively end many agents ability to sell space on the airline.

Currently, when a United ticket is purchased through a travel agency, the agent charges the card on behalf of United Airlines, and adds a service fee through the Airlines Reporting Corp (ARC), the clearing house for airline and travel agency ticketing transactions.