The company formerly known as BAA is to sell Stansted airport to the Manchester Airports Group for £1.5bn.
The deal will mean that BAA - now known as Heathrow Airport Holdings - will be responsible for just four UK airports compared with its original seven.
The remaining ones are Heathrow, Southampton, Aberdeen and Glasgow.
Colin Matthews, chief executive of Heathrow, said: "Stansted Airport and its people have been part of our company for a long time.
"It has been named by passengers as 'the world's best airport for low-cost airlines' for two consecutive years at the Skytrax World Airport Awards, and we are proud of its achievements.
"We wish the new owners every success and are confident the airport will continue to flourish.
"We will continue to focus on improving Heathrow, Glasgow, Aberdeen and Southampton airports."
Heathrow Airport Holdings has been forced to sell the airport after a ruling by the Competition Commission last year.
The company did not challenge the decision in the Supreme Court but maintained that the ruling "fails to recognise that Stansted and Heathrow serve different markets".
The sale of Britain's third busiest airport is expected to close by the end of February
Manchester Airports Group owns and operates Manchester, East Midlands and Bournemouth airports.
It had been one of the bidders for Gatwick when BAA put it up for sale but it lost out to American private equity group Global Infrastructure Partners which now also runs Edinburgh.