Obese passengers who are too large to fit into one seat face being charged for a second under new rules being imposed by Air France, and will not be allowed to board for “safey reasons” if they refuse to pay.

Overweight flyers will be charged 75 per cent of the cost of the second seat, which is the full price excluding tax and surcharges, on top of the full price for the first.

These passengers would be fully reimbursed for the second if the plane was not full.

However if they refuse to pay up front they will not be allowed on board for “safety reasons”.

At present Air France offers heavier passengers 25% off the price of buying an adjacent seat, a policy that has been in effect since 2005. The news regulations apply on bookings made after 1 February and all flights after 1 April.

Air France spokeswoman Monique Matze said: “People who arrive at the check-in desk and are deemed too large to fit into a single seat will be asked to pay for and use a second seat.”

“The decision has been made for safety reasons. We have to make sure that the backrest can move freely up and down and that all passengers are securely fastened with a safety belt,” she added

The average economy class seat on most Air France planes is between 43cm wide and 44cm wide, Ms Matze said.

She added: “People who cannot fit into a single seat will then be fastened by slotting the belt tip of one seat into the plug of the next, stretching over both seats.”

In 2008 Air France was ordered to pay £5,000 damages for “humiliation” of passenger weighing 27-stone who had his stomach measured at an airport check-in desk, and was then told he had to buy two seats.

British Airways has no weight limits for passengers, but advises overweight people to buy a second seat for their own comfort and safety if necessary.

In America, Southwest Airlines sparked outrage in 2006 when it announced that it would charging overweight people for two seats.