A large travel agency sued six major U.S. airlines this week, asking a federal court to bar the carriers from imposing limits on the commissions they pay travel agents.

Travel Network Ltd., which has 350 offices, sued American Airlines, Continental, Delta, Northwest, United and USAir in Minneapolis federal court.The suit marks the first significant legal action against the airlines since Delta started a broad move to cap travel agency commissions last Thursday. The other airlines copied Delta's decision within days and TWA joined them Wednesday, although it isn't named in the suit.

The suit seeks class-action status to represent all travel agents in the United States and accuses the airlines of a conspiracy to fix their commissions at "artificially low and noncompetitive levels."

It asks the court to declare the caps an illegal restraint of interstate trade and force the airlines to pay triple the amount of damages suffered by every affected agency.

"The action of the airlines requires a significant reaction from us," said Michael Brent, president and chief executive officer of Travel Network, based in Englewood Cliffs, N.J. "We certainly could not sit back and wait this one out."

Some of the airlines suggested the suit won't change their decision.

"Cost-cutting must be accomplished by an airline to survive," said Bill Berry, spokesman for Atlanta-based Delta. "From our standpoint, this lawsuit does not have any merit."

U.S. airlines have long paid commissions equal to 10 percent of the cost of a ticket. The new policy caps the commissions to a maximum of $50 for any domestic ticket over $500.

The suit says agents now issue about 85 percent of the tickets on the six airlines. The caps will reduce their commissions on at least 20 percent of the tickets they issue on those carriers, it says.

The average ticket costs a travel agent $29 to issue, so the agents only realize a profit on higher-priced tickets - the very ones that the airlines plan to cap, according to the suit.

Agents claim the caps will reduce profits and eliminate jobs.

"We were outraged by the action taken by Delta and then what followed," said Steve Loucks, spokesman for the American Society of Travel Agents, a travel trade association.

"It could put some agents out of business," Loucks said. "That will force the airlines to build up (their own ticket-selling operations) and that will drive up ticket prices for the customer."

Despite the pressure, Loucks said he doubts travel agents will start charging fees to fliers to make up for the lost income.

Herbert Teison, editor of the trade publication Travel Smart, said customers may suffer from the change in a more indirect way.

"If the travel agent stands to make a decent size commission, they're more accommodating," he said. "But now the agent's incentive is considerably reduced."

Still, some travel agencies are trying to work with the changes.

"We feel this move is going to stick and we are going to have to change our process to deal with it," said Travis Tanner, co-president and chief executive officer of Minneapolis-based Carlson Wagonlit Travel, the second-largest travel agency in the world.