NEW YORK (CNN/Money) - Passengers who are too large to squeeze between the arm rests of Southwest Airlines seats will be charged double for flying the low-cost carrier, the company said Wednesday.

The Dallas-based airline, which does not have first or business class sections with larger seats, said the policy has been in effect for some time but will be more strictly enforced beginning next week.

Starting next Wednesday, its "people of size" policy will require passengers who need seat-belt extensions or cannot lower the arm rests on their seats to purchase two seats if they are flying on a plane near or at capacity.

"If you consume more than one seat, you will be charged for more than one seat," spokeswoman Beth Harbin said.

Harbin said that under the existing policy, fewer than 1 percent of Southwest passengers have been asked to buy a second seat, which is offered at the same rate at which the passenger purchased the original ticket.

Southwest spokeswoman Christine Turneabe-Connelly said customers buying tickets online or over the phone will have to purchase two seats on their own accord. She added that information on the two-seat policy for passengers of size is available on the company's Web site and can be obtained from agents over the phone.

If a passenger of size buys only one seat online or over the phone, gate or ticketing agents at the airport will "discreetly" inform the passenger that it is Southwest's policy that an additional seat must be purchased, Turneabe-Connelly said.

Agents will use their judgment to decide if passengers require two seats, she said. If a passenger insists only one seat is required, the agent will "address that situation explaining that a second seat is for their safety and the safety of those around them," she added.

Southwest seats are 18-3/4 inches wide.

When the plane is not crowded, a larger passenger can apply for a refund for the second seat, Harbin said.

Southwest said that about 90 percent of the letters it receives on the issue have been from passengers complaining that their seating has been encroached upon by larger neighbors. The airline said one of the top complaints it receives is from passengers who say they were "sat on" during their flight.

Fat activist Marilyn Wann, author of the book "FAT!SO?," said airlines should provide seating that accommodates people of all shapes and sizes who are paying for a trip.

"You are buying passage from point A to point B. You are not buying real estate," Wann said. "I encourage people of all sizes not to encourage this kind of treatment."

The airline was sued eight years ago on the issue by a larger passenger forced to buy a second seat and the case was dismissed, Harbin said.

The airline said the move to more strictly enforce the policy was prompted by its decision to stop giving out its trademark plastic boarding passes on a first-come, first-served basis to passengers at its departure gate.

Southwest (LUV: up $0.69 to $15.84, Research, Estimates), which does not have assigned seating, is starting to dispense paper boarding passes instead in an effort to cut waiting time for passengers who have to pass through more stringent security measures after the Sept. 11 attacks.

The new boarding passes, which are given out at skycap counters, ticket gates and departure gates, provide an opportunity for Southwest workers to show greater discretion in enforcing the people of size policy, airline officials said.

Morgan Downey, executive director of the American Obesity Association, said his organization is considering lawsuits against the carrier for a policy he called highly subjective.

"They are packing us in like sardines and they say it's our fault that their seats don't fit the traveling public," he said.