Air France-KLM is getting heavy on overweight passengers.

If passengers are so obese people they cannot squeeze into a single plane seat, they will have to pay nearly double to fly, the company says.

Spokeswoman Monique Matze says extra-large flyers will have to pay 75 per cent of the cost of a second seat (the full price excluding tax and surcharges) on top of the full price for the first, AFP reports.

She says the decision was made for safety reasons, as seat backrests need to be able to move freely up and down and all passengers must be secure with the safety belt.

People who cannot fit into a single seat are fastened by slotting the belt tip of one seat into the plug of the next, stretching over both seats.

Paying for both seats will ensure they are next to each other, but if flights are not fully booked overweight passengers will be refunded the extra charge.

The new measure applies to bookings from February 1 for all flights from April 1.
Plane seats average 43cm wide and 44cm for long-haul flights.