Airports operator BAA lost its appeal at a tribunal today against a ruling that it must sell Stansted Airport, the Competition Commission said.
The defeat marks the latest stage in legal wranglings which have been rumbling on since 2009 when BAA was ordered by the Commission to sell a number of its airports.
The Commission's original ruling found there were problems which adversely affected passengers and airlines at all seven BAA airports, which at the time included Gatwick.
But BAA claimed there had been material changes since the Commission first made its decision - including its sale of Gatwick for £1.5 billion to Global Infrastructure Partners.
A BAA spokeswoman said: "We are disappointed by the decision of the Competition Appeal Tribunal, which we will now carefully consider before making any further statements."
When the Commission's investigation started in 2007, it was feared that BAA's strong presence in the South East could slow any future development of runways.
BAA said the Government's decision not to build any more runways in the South East for the foreseeable future should have a bearing on the original decision.
The airport operator currently has Edinburgh Airport on the market, which it hopes to have sold by the summer.
Stansted saw 18.3 million passengers pass through its doors in 2011, making it the fourth busiest airport in the UK. Passengers can fly to 31 countries from the airport, which sees 550 aircraft movements per day.
The airport has 10,200 employees, of which 1,420 are BAA Stansted staff.
Laura Carstensen, chairman of the BAA remedies implementation group and a member of the original inquiry group 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.
"BAA is in the process of selling Edinburgh airport following our ruling. We will now press ahead to ensure that this is followed promptly by the sale of Stansted airport, to the benefit of passengers and airlines."