It is understood that global credit rating agencies are preparing to downgrade the BAA bonds after last week’s terror threats, which could lead to higher security costs and lower duty-free revenues.

BAA, already under fire from its biggest airline customers and the travelling public, will be forced to wrangle with bondholders over the level of coupon on its debt as the investors seek to take advantage of the changing nature of air travel resulting from new security measures.

To add to the pressure on Ferrovial, a bridging loan that was used to finance the acquisition will become more expensive six months after completion of the takeover. Rates will rise prohibitively from July, according to sources familiar with the deal.

The Spanish construction group, which paid £10.3 billion last month for Britain’s busiest airports, including Heathrow, Gatwick and Stansted, took the first steps towards a refinancing of BAA’s £4.7 billion bonds last Thursday, the day strict new security measures were introduced at Britain’s airports, leading to thousands of flight cancellations and long queues of passengers.

It has asked the Association of British Insurers to facilitate the negotiations and has begun an “initial holdings inquiry” to establish information about the outstanding bonds and their holders.

Richard Bartlett, a Royal Bank of Scotland banker who is involved in the refinancing with Citigroup, said that the company intended to refinance the bonds on a longer-term basis that was more akin to the water industry. The talks were considered extremely sensitive and could involve lengthy and complex negotiations, he said.

Alex Moss, deputy head of credit research at Insight Investment, the asset management arm of HBOS, said: “I think that they should do something sooner rather than later. There was a lot of uncertainty during the bidding process and there is still concern about the price they paid. It was a very full price.”

The Spanish infrastructure group is expected to fund the business by raising debt secured on future cashflows. However, those cashflows could be hit severely by higher security costs and permanent changes to the amount of hand baggage people can carry on to flights, which are expected to have an impact on sales at BAA’s World Duty Free subsidiary.

John Hatton, a debt analyst at Fitch Ratings, said: “We would take into account the forecasts for levels of spending going forward. If the level of spending is fundamentally changed as a result of these security measures, we would have to take that into account.”

Fitch has had BAA senior unsecured debt on “rating watch negative” since June, when the consortium led by Ferrovial secured the group after a bid battle.

BAA declined to comment.

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