United Airlines Inc. and Orbitz Worldwide LLC sued to prevent the travel website Skiplagged.com from helping consumers buy what the companies call improper “hidden city” plane tickets that undercut their sales.

The companies accused the site’s founder, Aktarer Zaman of New York, of “intentionally and maliciously” interfering with their airline industry business relationships “by promoting prohibited forms of travel,” in their complaint filed today in federal court in Chicago.

Skiplagged helps consumers find cheap airfares by enabling them to book passage on a flight with one or more stops and then deplane before the flight reaches its as-booked final destination.

“In its simplest form, a passenger purchases a ticket from city A to city B to city C but does not travel beyond city B,” according to the companies’ complaint. “‘Hidden City’ ticketing is strictly prohibited by most commercial airlines because of logistical and public-safety concerns.”

Among those concerns, according to the complaint, is United’s resultant inability to estimate flight passenger counts which can cause departure delays and affect fuel load computations.

Orbitz, an online travel booking site, and United claim neither of them gave Zaman permission to engage in hidden-city ticketing. Claiming he is unfairly competing against them and creating false associations by linking customers to their websites, they’re seeking a court order halting the conduct.

Zaman, reached via LinkedIn.com e-mail, declined to immediately comment on the suit.

United Airlines is a unit of Chicago-based United Continental Holdings. Orbitz is a unit of Orbitz Worldwide Inc.

The case is United Airlines Inc. v. Zaman, 14-cv-9214, U.S. District Court, Northern District of Illinois (Chicago).

To contact the reporter on this story: Andrew Harris in federal court in Chicago at aharris16@bloomberg.net. To contact the editors responsible for this story: Michael Hytha at mhytha@bloomberg.net.

Photo Credit: In order to save money, Skiplagged sometimes books flights to destinations people aren't going to. Above is a roundtrip flight from New York to Los Angeles that includes an additional leg to Oakland that the passenger will not take.