A group of passengers and travel agents filed a lawsuit in San Francisco Monday, challenging a rule change on "fare combinability" that allows the nation's largest airlines to charge "hundreds and even thousands of dollars more for multi-city itineraries than if the same flights were purchased separately."

The suit, filed by the Joseph Alioto firm, said the actions to eliminate discounted fares and set fare restrictions for tickets purchased for multi-city air travel within the United States constitutes a conspiracy and violates antitrust law.

Flying to more than one city on a single ticket fare recently became more expensive when the nation's three largest airlines blocked their least expensive fares from being combined on a multi-city trip.

American, Delta, and United, which control more than 70 percent of the U.S. market, made it more expensive on April 1 to book a multi-city itinerary -- for example, from Philadelphia to St. Louis to Chicago and back to Philadelphia -- compared to purchasing each leg of the trip separately.

The airlines eliminated the "combinability" of lower non-refundable to create a connecting itinerary. The lawsuit contends the airlines' actions mean higher prices, and higher fees. If passengers book each flight separately to get the lowest prices on trips with multiple stops, they will be charged $200 in change fees for each leg of the trip, if they later change their travel plans, the suit said.