BAA could lose up to £1 billion if it is forced by the Competition Commission to sell three of its airports.

Analysts say that airport valuations have slumped since Ferrovial, the Spanish infrastructure company, bought BAA for £10.3billion two years ago. That was at the height of the property and takeover bubble. Since then credit has become more expensive and there has been a downturn in the aviation sector.

According to estimates by City analysts, Ferrovial paid approximately £4 billion for Gatwick, Stansted and either Edinburgh or Glasgow airport in 2006. Gatwick was the most valuable, worth £2 billion, while Stansted cost about £1.5 billion. However, based on present valuations for infrastructure assets, these airports would be worth just over £3 billion now, representing a huge loss for Ferrovial.

Ashley Steel, head of KPMG'S infrastructure practice, said: “Shareholders will be naturally anxious, and timing will be critical - as will the sale process - to ensure that a good and fair price is achieved.”

The Competition Commission said yesterday that if its final report, due in March, required divestment of the airports, BAA may be required to act by the end of 2009.

Douglas McNeill, transport analyst for Blue Oar Securities, said: “The commission's proposals could hardly have been bolder - and herald the biggest shake-up of British aviation for decades.”

Ferrovial's acquisition of BAA has seriously damaged the Spanish company. Its share price has fallen by 50 per cent since the deal and it has been paying €1.9 billion (£1.5 billion) a year to service its debt. Ferrovial, along with other partners, also had to inject £400 million into BAA this year after delays to a refinancing programme.

Its share price initially rose 3 per cent in Madrid yesterday as investors were optimistic about reducing the company's debt. The stock then lost these gains as analysts began to calculate how much less BAA might get for its assets.

Colin Matthews, the BAA chief executive, insisted that the airports had retained their value: “If we are required to sell the airports, we will do so. These assets are attractive and valuable because we have secured their future.”

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