United Airlines and travel firm Orbitz have launched legal action against a site that seeks out cheap "hidden city" airfares.

The site finds cheap fares by looking for flights that have a stopover at the city someone wants to travel to.

The two firms allege the site is engaged in "unfair competition" and seeks to recoup lost revenue.

The developer behind the site said he was doing nothing wrong by exposing the "inefficiencies' in airline ticketing. The legal action has been filed in Illinois.

The Skiplagged website works by looking for longer flights that include a stop in a big city en route to another destination. One example might be flying from New York to Lake Tahoe that has a stopover in San Francisco.

If someone wanted to travel to San Francisco they might spend less on the fare by booking the stopover flight and not travelling to Tahoe than they would simply booking a flight to San Francisco from New York. In some cases, the site suggests, travellers can save 40 percent or more on ticket fares.

The trick only works with one-way flights. Travellers cannot check in any luggage as that would then travel on to the flight's final destination.

Twenty-two-year-old developer Aktarer Zaman, who created the site, told CNNMoney that he had made no profit from Skiplagged. He declined to comment specifically on the case to CNN.

Zaman has launched a fundraising campaign to gather cash to fight the legal battle against United and Orbitz. So far he has raised $10,538 (£6,776) of the $15,000 needed.

In its legal filing. United and Orbitz said the site was "intentionally and maliciously* interfering with the travel firms' business and was making it breach its contracts with its partners.

The documents added that "logistical and public safety concerns” meant using "hidden city" tickets was prohibited and, as a result, using Skiplagged broke these rules.

The two firms are seeking damages of at least $75,000 in revenue they claim they have lost as a result of Skiplagged operating.