AirTran of the United States has become the latest airline to charge obsese passengers for a second seat, mimicking the line of parent Southwest Airlines.

“Customers of size” will be forced to pay double for their travel from March next year.

However, relatively few passengers are likely to be hit by the charge, with Southwest Airlines arguing “less than one per cent” of passengers are likely to be in line for the cost.

Check-in staff and flight attendants will be given the delicate task of identifying those who cannot sit in a single seat

Economy seats are 18 inches wide in Air Tran economy class.

Customers flying with parent Southwest Airlines – which acquired Air Tran earlier this year – are warned the carrier “became more vigilant regarding the additional purchase when we began seeing an increase in the number of valid complaints from passengers who travelled without full access to their seat because a large customer infringed upon the adjacent seating space”.

If there are remaining seats on the plane, plus-size passengers who have been charged for an extra seat will be entitled apply for a refund.