The Department for Transport is worried that its airports strategy, which depends on BAA’s close co-operation, would be undermined by a change of ownership. BAA is careful never to challenge government policy openly in return for a tacit commitment by ministers not to tamper with the company’s near monopoly on airports in the South East of England. In addition to Stansted, BAA owns Heathrow and Gatwick airports.

BAA has faced tough questions from shareholders about its plan to spend £2.7 billion at Stansted, without making clear how it would fund the investment. The greatest demand for a new runway is at Heathrow, yet BAA has said that it plans to build one at Stansted first. It plans to open the new Stansted runway by 2013, possibly followed by a new runway at Heathrow between 2015 and 2020.

The Civil Aviation Authority (CAA) opposes any cross-subsidy for Stansted from airlines operating at Heathrow, yet it remains unclear whether BAA could fund the expansion purely by increasing charges at Stansted.

Stop Stansted Expansion, the residents campaign group, said: “Nobody pays a premium to take over a company unless they think they can add significant value by running the business differently. BAA needs to make up its mind if it’s a company operating in the interests of the public or its shareholders.”

A Department for Transport source said: “We are following events closely as we have a strategic interest in the outcome. Different companies can take different views about major investments.”

It remained unclear last night if the Government had powers to intervene in a sale of BAA, given the importance of Heathrow to the UK, and whether the CAA could veto any potential suitor on grounds of competence or suitability. The Government, which privatised BAA in 1987, no longer owns a stake in the airports group and three years ago redeemed a “golden share”, which effectively protected BAA from a takeover.

A spokesman for the CAA would only say that the authority could regulate designated airports “regardless of the owners of the airport”.

The influence of British Airways, the dominant carrier at Heathrow, with 41 per cent of all take-off and landing slots, could also play a key role. A spokesman for BA declined to comment.

A takeover by a Ferrovial-led consortium remained far from certain. The Spanish group was known to be frustrated at having to declare its interest at such an early stage, before even making a formal approach to BAA’s board.

Macquarie Bank, the acquisitive Australian investor that jointly owns Bristol airport with Ferrovial, was seen as a possible consortium member, as was the French group Vinci.

Abertis Infraestructuras, the Spanish group that paid £551 million for TBI, the Luton airport owner, in 2004, ruled out joining the consortium.

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