An Air France spokesperson says obese passengers who are asked to pay for an additional seat will be reimbursed if the plane isn't full.

Air France-KLM yesterday announced that passengers who were unable to squeeze into a single aeroplane seat would have to pay 75 per cent of the cost of a second seat.

Company spokeswoman Monique Matze told AFP the new measure was being introduced for "safety" reasons and would come into effect on all flights from April 1.

Air France and KLM merged in 2004 but both airlines continue to fly under their own brand names.

The additional charge will apply for any post April 1-tickets booked from February 1 onwards.

However Air France, which has long offered obese passengers the option of buying an adjoining seat at a discount, said passengers would be fully reimbursed for the second fare in 90 per cent of cases.

Obese passengers who don't reserve a second seat may not be allowed to board, at the captain's discretion and if there is not an unoccupied adjoining seat.

"It's a question of security," Air France spokesman Nicolas Petteau said.

The airline today denied reports in the French press that it would oblige obese passengers to buy a second seat.

"It is not an obligation; we suggest to such passengers they buy a second seat for their own comfort and in order to be sure the seats are adapted to their needs," Air France spokesman Jean-Pierre Lefebvre told Reuters.

An Air France statement emailed to nzherald.co.nz said the airline had been offering "corpulent passengers the possibility of purchasing a second seat to ensure they travel in optimum comfort and safety" since 2005.

"This second seat benefits from a 25 per cent discount," the statement said.

The only change stemming from yesterday's announcement would be that - from February 1 - heavier passengers would be refunded the cost of the second seat they had purchased, provided the Economy cabin was not fully booked, the statement said.

Three years ago Air France was sued by a 160-kilo passenger who the airline obliged to buy a second seat for a full New Delhi-Paris flight.

Air France was ordered to pay €8,000 ($15,688) in damages and to reimburse the cost of the second seat.

Other airlines with similar policies on obese passengers include Southwest, JetBlue and American Airlines.

New Zealand aviation commentator Peter Clark believes a "fat tax" on airlines is the way of the future.

Mr Clark told Newstalk ZB the concept was initiated by airlines in America and is slowly being introduced by other airlines around the world.

"We're seeing a trend now that airlines are saying, hold on, we're carrying extra weight with more people. We have safety issues with larger people. It impedes smaller people in the plane and costs more in fuel to carry heavier people."

Mr Clark says airlines may have to consider widening seats, but that would push up the cost of flying.

- NZ HERALD STAFF, AP, NEWSTALK ZB