Airport operator BAA has lost its appeal against a ruling that it must sell Stansted airport.
The Competition Commission first ruled three years ago that BAA's dominance in London and Scotland meant it must sell Gatwick, Stansted and either Glasgow or Edinburgh airports.
BAA sold Gatwick and recently agreed to sell Edinburgh, but it has continued to fight the Stansted decision.
The firm said it was "disappointed" and was considering its position.
Its appeal was dismissed by the UK Competition Appeal Tribunal, a judicial body whose panel is made up of judges and industry experts.
The Spanish-owned company will be unhappy about having to sell Stansted in such an unfavourable economic climate, BBC transport correspondent Richard Westcott said.
BAA argued that it was not reasonable to consider Stansted as serving the same market as its other London airport, Heathrow, as they serve different markets and are used by different airlines.
As a result, BAA said it was not anti-competitive for it to operate both airports, but the competition authority did not agree.
Responding to the tribunal's ruling, Laura Carstensen, a member of the original Competition Commission inquiry which reported in 2009, said: "We are very pleased that our decision has been upheld.
"Whilst BAA is of course entitled to explore the available avenues for challenge, it is now surely time for BAA to accept our findings and proceed with the necessary divestments."
Speaking in July last year, BAA chief executive Colin Matthews said: "The Competition Commission has not recognised that the world and BAA have changed.
"A new government has changed aviation policy to rule out any new runway capacity in the South East and BAA has sold Gatwick Airport.
"Both are significant changes to the airport market. Further, the airports in question face increased competition from non-BAA airports, particularly those in Europe, for the business of low cost carriers who now take a pan-European view of the market."