The major airline is set to make the introduction from January onwards.

Southwest Airlines will make the policy introduction from the beginning of next year.

They also plan to switch from open seating to assigned seats, which marks a huge departure from their decades-old model.

The 'customer of size' policy once let passengers buy a second seat for comfort and then get their money back after the flight, but with the new conditions, the refund won't come so easily.

Southwest has said that criteria now must be met in order for passengers to qualify for a refund.

Conditions include that the flight must have had at least one open seat, both tickets must be purchased in the same fare class, and the refund request must be submitted within 90 days.

Critics have said that the change will make air travel harder and more expensive for passengers who don't fit into a standard seat.

Jeff Jenkins, founder of the plus-size travel community Chubby Diaries, told USA Today: "It seems like a sneaky add-on from a policy that had been around for 20+ years without much hitting the bottom line."

"It's just more anxiety to an already high-anxiety experience."

Furthermore, under the new rules, passengers who show up without booking an extra seat could be put on a later flight and forced to pay a walk-up fare for the extra space, often more expensive thanbooking in advance.

A seat on a Southwest flight is now described as the space between two lowered armrests, and it says that anyone who can't fit in that space comfortably is required to purchase two seats.

The airline added that the policy, which affects around 0.25% of passengers, is part of a wider remodel.

Head of the National Association to Advance Fat Acceptance, Tigress Osborn suggested that the changes are already affecting the plus size community and says many won't be able to risk not getting their money back.

In an Insatgram post, she wrote: "They can't afford to take that gamble.

"This will literally mean they cannot travel by air anymore."

She further warned: "Now that people are choosing seats, you need to buy your tickets as early as possible.

"But even then, there's no promise they won't move your seats."

Southwest has yet to confirm whether adjoining seats will be ensured and has only said they will 'attempt' to rebook passengers on a later flight if needed.

Additionally, Corinne Fay, who runs the fashion newsletter Big Undies, dubbed the refund policy as the only thing keeping Southwest affordable.

"I definitely felt like crying because it'll make my life a lot harder,' she said. 'We're talking about the difference between spending $400 versus $800 or even more," she said.