Now it has become a financial issue as well.

If you consume more than one seat, you will be charged for more than one seat

Southwest

US budget airline Southwest has said people will be charged twice if they are too large to fit into a standard seat.

Overweight passengers in need of a seat-belt extension, or with a girth that makes it impossible to lower the arm rests between seats, will have to buy a second ticket if the plane is fully booked.

Southwest flies exclusively Boeing 737 planes with six seats in each row.

It operates no business or first class sections with wider chairs, and seats are not reserved but allocated in the cabin on a "first come, first served" basis.

A slim revolt

Southwest's "passengers of size" policy is not new. It actually dates back to 1980, although rarely has it been enforced.

But Southwest's slim-line passengers are now revolting. The airline says 90% of the letters it receives on the issue were from passengers complaining that they were "sat upon" by people overflowing from their seat.

Executives at Southwest's Dallas headquarters saw only one solution. "If you consume more than one seat, you will be charged for more than one seat," said spokeswoman Beth Harbin.

Wider than 48 centimetres?

There is just one consolation for "passengers of size": they will get the same discount on the second seat that they received for the original ticket, which can amount to up to 55% per seat.

And if the plane stays fairly empty, two-ticket holders can apply for a refund.

I encourage people of all sizes not to encourage this kind of treatment

Marilyn Wann, author of the book "FAT!SO?

Southwest's Ed Stewart says it's all about "safety and comfort for everyone".

Southwest's seats are 48 centimetres (18-3/4 inches) wide.

"If you buy a seat you're entitled to a seat and not necessarily a seat-and-a-half or two seats."

Discriminating against a health problem?

Not everybody agrees.

Some passengers at Southwest's check-in desks queried about the policy argued that if passengers were "that big, they have a health problem", and people with health problems should not pay extra.

Marilyn Wann, author of the book "FAT!SO?, has weighed in to the debate, arguing that a ticket entitles passengers to get from "point A to point B, they are not buying real estate".

"I encourage people of all sizes not to encourage this kind of treatment."

But Southwest is not the only carrier with special rules for wide-bodied customers.

Many traditional airlines with reserved seats already require extra-large passengers to purchase a second seat in advance.

Larger passengers will find it difficult to fight the two-ticket policy.

Eight years ago, a passenger sued Southwest after being forced to buy a second seat, but in court the case was dismissed.