Airport operator BAA has warned shareholders that it has already spent £15m trying to fend off a hostile approach from Spanish contractor Ferrovial, and that costs are likely to rise further.
In its annual results announcement on Tuesday, BAA said: "The total costs to the company of this ongoing situation are, at this stage of the process, uncertain but will be significantly greater than £15m."
Ferrovial has made a formal £8.75bn bid for BAA which the board has rejected.
Goldman Sachs has indicated that it is considering a £9.5bn bid which BAA has said it would also reject.
The UK Civil Aviation Authority has voiced concern that the debt incurred by potential buyers of BAA may mean that they are unable to invest properly in airport services.
In the year to 31 March, operating profit at BAA rose 8.1% to £710m on a £2.23bn turnover, up 7.4%.