A major U.S. airline said this week it would begin charging passengers who cannot fit into a single seat for a second space on its flights.

United Airlines said men and women who cannot squeeze into an existing seat on sold-out flights will be told they either need to wait for another plane with empty seats or purchase a second seat to accommodate their bulk.

The new rules apply to anyone flying after April 15.

Basically, if you cannot buckle your seatbelt or put down your armrest, a United Airlines flight just became more expensive, according to the new guidelines.

"If no unused seats are available on the ticketed flight, then the customer must either purchase an upgrade to a cabin with available seats that address the above-listed scenarios, or change his or her ticket to the next available flight and purchase a second seat in addition to the one already purchased," said United Airlines' Web site.

United Airlines said it implemented the policy to align itself with what other airlines do.

A weighty problem

The obese passenger issue has been a hot button with air travellers for years. And flying sites are rife with passenger complaints about hefty flyers taking up too much room on an aisle.

"I was immediately squashed into the window. I was mad; I didn't pay to be smothered for five hours," griped one traveller in a story on the Flightsfromhell.com website.

But, governments have been more sympathetic to the plight of the overweight traveller.

In 2008, the Canadian Transport Agency, the federal organization which regulates the country's airlines, ruled carriers would need to accommodate people with weight issues rather than force the person to buy another seat.

Air Canada and WestJet both fought the order to the Supreme Court of Canada but ultimately lost their case and have been providing seats according to the new so-called "one-person-one-fare" rules since January.

The two carriers, however, still require a note from a doctor indicating that the passenger is obese.

In the United States, the situation appears to be less clear.

The U.S. Federal Aviation Administration does not allow air carriers to discriminate against disabled individuals, except in very specific circumstances.

"Carriers shall not exclude any qualified individual with a disability from any seat in an exit row or other location or require that a qualified individual with a disability sit in any particular seat, on the basis of disability," according to U.S. Department of Transportation.

The problem is that the U.S. rules do not appear to list excessive weight as a "disability."