SAN FRANCISCO (KPIX 5) — Three of the nation’s largest airlines are accused of illegally enacting a little-noticed ticket-pricing policy that could add hundreds of dollars to the cost of your next vacation.

San Francisco attorney Joseph Alioto says back on April 1, United, American and Delta all stopped automatically charging the lowest fares on each leg of a multi-city flight.

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For example, in its announcement of the new fare changes, United demonstrated that, under its previous pricing system, two one-way flights from Los Angeles to Houston and Houston to New Orleans would cost a combined total of $189.

However, its new fare rules would require that passengers who want to travel from Los Angeles to New Orleans instead book one ticket for $363.

Alioto says that each of the airlines did the same thing at virtually the same time, and using the same language, shows they were colluding.

“It’s price fixing,” Alioto told Consumerwatch.

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“If they did it individually – They never would because the other would be able to take their customers away,” Alioto said.

Why the change?  Alioto says airlines just weren’t making enough money the old way.  “They discovered that some legs were so low because those were in areas in which they were competing with low cost carriers,” Alioto told Consumerwatch.

The law suit, filed on behalf of travel agents, says the airlines threated to charge agents the difference in fares if they continued booking the cheaper individual legs.

American airlines calls the lawsuit “completely without merit and says the move simply “eliminated a loophole.”  Meanwhile United said it “makes changes to its prices or fare rules unilaterally to ensure competitiveness”

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Delta declined to comment on pending litigation.