About 35 million passengers passed through Gatwick in 2007
The Competition Commission has said it will require BAA to sell Gatwick, Stansted and Edinburgh airports.
The watchdog says the decision is subject to a final consultation but is necessary to improve levels of service at UK airports.
In August, the watchdog had said BAA may have to sell three of its airports because of a lack of competition.
BAA said that it did not believe the Commission had "compelling evidence" that Stansted should be sold.
It added that there was "no justification" for specifying which Scottish airport should be sold.
"We do not believe that it has set out compelling evidence to support its view that selling Stansted as well as Gatwick will increase competition and we remain concerned that its proposed remedies may actually delay the introduction of new runway capacity," BAA said.
BAA, which runs seven UK airports, was bought by Spanish firm Ferrovial for more than £10bn in 2006.
The Competition Commission said it would also propose measures to ensure that service and investment at Heathrow meets the needs of airlines and passengers.
2007 PASSENGER NUMBERS AT BAA AIRPORTS
Heathrow: 67.8 million
Gatwick: 35.1 million
Stansted: 23.7 million
Edinburgh: 9.0 million
Glasgow: 8.7 million
Aberdeen: 3.4 million
Southampton: 1.9 million
"Under separate ownership, the airport operators including BAA, will have a much greater incentive to be far more responsive to their customers, both airlines and passengers," the Competition Commission said.
BAA had put Gatwick up for sale following the Competition Commission's initial report in August.
Several firms are said to be interested in buying Gatwick, which has been valued at £1.8bn by regulators.
Potential bidders include Australian company Macquarie, Germany's Fraport, and the owners of Manchester airport.
Virgin Atlantic has said it is preparing a bid as part of a consortium.
The Competition Commission said it would consider responses to its provisional decision and it would publish its final report in late February or early March.