The airport operator BAA must sell Stansted and either Glasgow or Edinburgh airports, the Competition Commission has said in its final ruling.
The sale process must start in three months "or sooner if undertakings are accepted from BAA in the meantime", the commission said in the report.
The commission believes that passengers and airlines will benefit from greater competition between airports. In the report they said that separate ownership would help, "despite the current government’s decision to rule out new runways at any of the London airports".
Peter Freeman, the CC's chairman of the BAA remedies implementation group, said:
"We hope that the sales can now proceed without delay so that passengers and airlines can start to enjoy the benefits of greater competition. Our report has been challenged, reviewed and upheld and it is clear that the original decision to require BAA to divest three airports (including Gatwick) remains the right one for customers."
The decision seemingly ends a two-year battle in which BAA has fought to retain the bulk of its UK business.
The commission first told BAA, owned by the Spanish company Ferrovia, that it must sell some airports in March 2009.
BAA appealed the Competition Commission's decision and won a victory in early 2010 through the Competition Appeal Tribunal. However last October the original order was restored by the Court of Appeal.
BAA sold Gatwick in October 2009 for £1.5bn. The company still operates Heathrow, Aberdeen and Southampton airports.
Colin Matthews, chief executive of BAA, told the BBC's Radio 4 Today programme on Tuesday that the ruling was "unreasonable" and "draconian". He said that the company had invested £5bn in UK jobs and hinted that the company may continue the fight in the European courts.
Matthews added that BAA "haven't decided" if they will sell Edinburgh or Glasgow airport.