The airline has confirmed it's changing its policy on larger passengers revealing they still need to book an extra seat in advance, but they are no longer guaranteed a refund and they won't find out whether they are eligible to get their money back until they fly because the airline will only hand back their cash if the section they have booked isn't full on departure.

Now the change has been criticised by travel expert Jeff Jenkins, founder of Chubby Diaries which provides tips and advice for plus-size travellers, who told USA Today: "It seems like a sneaky add on from a policy that had been around for 20 plus years without much hitting the bottom line.

"I just hope that consumers are aware of this change and I wonder if plus size people will skip out on flying with them at all because of them not knowing if the flight is sold out or not.

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

The change will come into affect from January 27 - when the airline will also introduce assigned seating.

A statement from Southwest Airlines explained the changes: "To ensure space, we are communicating to Customers who have previously used the extra seat policy that they should purchase it at booking ...

"Customers who encroach upon the neighbouring seat(s) should proactively purchase the needed number of seats prior to travel to ensure the additional, adjacent seat is available."

The airline added the conditions that must be met to obtain a refund include the flight "must depart with at least one open seat or with passengers travelling on space available passes", "both seats should be purchased in the same fare class" and "the refund request must be made within 90 days of your date of travel."

Jason Vaughn, who posts tips for plus-size travellers on his website Fat Travel Tested, told CNN: "I think it’s going to make the flying experience worse for everybody ... [Southwest Airlines] have no idea anymore who their customer is. They have no identity left."