BAA must sell Stansted and either Glasgow or Edinburgh airports, the Competition Commission has ruled.
The Competition watchdog ruled that BAA, owned by the Spanish infrastructure firm Ferrovial, exerted a dominant hold on UK airports and as such, it must shed its assets.
In March 2009, the commission told BAA to sell Gatwick and Stansted airports and either Edinburgh or Glasgow.
BAA sold Gatwick but challenged the decision to sell the Scottish hubs.
BAA, which operates Heathrow, Southampton and Aberdeen, as well as Stansted, Glasgow and Edinburgh, said it was dismayed at the decision and would now consider a judicial review.
Following the release of its report next week, the Competition Commission said that BAA must start selling Stansted airport within three months and it should be followed by the sale of one of the Scottish airports.
BAA is expected to opt to sell Glasgow airport due to the stronger recent performance of Edinburgh. The official deadline for the sale of Stansted given to BAA by the Competition Commission will not be made public although the regulator will insist that Stansted be sold off first
When BAA challenged the commission’s initial ruling in October last year, the Court of Appeal ruled against the airport operator. Earlier this year, the Supreme Court refused BAA permission to appeal further.
"Our report has been challenged, reviewed and upheld and it is clear that the original decision to require BAA to divest three airports remains the right one for customers," said Peter Freeman from the Competition Commission.
The commission said its decision was "fully justified" and passengers and airlines "would still benefit from greater competition with the airports under separate ownership, despite the current government's decision to rule out new runways at any of the London airports".
But BAA chief executive Colin Matthews called the decision "an unreasonably draconian demand".
"The world has changed since that [initial] report more than two years ago. It's more clear than ever that Heathrow does not serve the same market as Stansted," he told BBC Radio 4's Today programme.
"Any reasonable and legal way that we have to protect the company which has invested £5bn in UK jobs, we will do."
Budget airline Ryanair accused BAA of "using delay tactics to maximise the amount it can raise for the inevitable sale".
"This is a cynical move which will damage London tourism and traffic and keep costs high for passengers," a spokesperson for the airline said.