BAA will be forced to offload Stansted and either Edinburgh or Glasgow airports after the Competition Commission (CC) issued its final decision on the matter.
It reaffirms the commission's provisional view published in March and will lead to the sale process starting in three months time, with Stansted to be sold off first.
Peter Freeman, chair of the BAA Remedies Implementation Group, said: "We think the benefits will be all the greater once Stansted, Gatwick and Heathrow are all in competition with each other.
"There has also been no cause to alter our view on the need for either Edinburgh or Glasgow to be under separate ownership."