Airbus is standing firm on its plans to prevent any conversion between the baseline A320 and the new A320neo.
This is despite expectations from AirAsia that it will be able to switch part of its backlog to the re-engined twinjet.
Airbus chief operating officer for customers John Leahy, speaking in Toulouse today, said that allowing conversion "creates too much confusion".
He says the airframer "needs to know" how many aircraft of each type it will be dealing with as it makes the production transition.
A customer with a conversion capability is "really taking two slots", Leahy says: "And we're not going to do that."
Airbus executive vice-president for programmes Tom Williams adds that the airframer "doesn't want to cannibalise the backlog".
Having secured around 300 commitments to the A320neo family, Airbus is expecting this figure to increase to 500 by around June.