Airports operator BAA has lost its three-year battle against a Competition Commission ruling to sell Stansted airport.
The appeal was rejected by three Court of Appeal judges sitting in London and will force BAA to sell after a series of unsuccessful challenges since the ruling in 2009.
The Competition Commission welcomed the decision. Laura Carstensen, chairman of the BAA Remedies Implementation Group and a member of the original Inquiry Group, said: "It remains the right decision in the interests of passengers and airlines and it is surely now time for BAA to accept the verdict and proceed with the sale."
The decision comes after the Spanish-owned airports operator lost an appeal at a Competition Appeal Tribunal in February. BAA has already sold Gatwick and agreed to sell Edinburgh.
The decision comes at a time when Stansted is losing passengers, with a 4.5% drop to 8.2 million in the half year to June.
BAA chief executive Colin Matthews had argued that when UK consumer confidence picks up, Stansted will be "growing faster than any other aiport" and that its sale would have a damaging impact on shareholders.