Spain's Grupo Ferrovial is considering a bid for British airport group BAA but has not yet approached the UK firm about any takeover approach.
Ferrovial already has interests in Bristol and Belfast airports
BAA operates Heathrow, Gatwick, and Stansted airports in London; Glasgow, Edinburgh, Aberdeen, and Southampton.
The UK firm said it had not received any proposal from Ferrovial and "strongly advises shareholders not to take any action at this time".
However, BAA's shares rose 15% on the news of the possible approach.
Ferrovial said any offer would be in cash and most likely in a consortium.
"No approach has yet been made to the board of BAA," the Spanish firm added.
The announcement also said Ferrovial's considerations were at a preliminary stage and that it did not follow necessarily that any formal offer would be made.
BAA shares soared 20% following the announcement from Ferrovial, which already owns half of Bristol airport and completely owns Belfast City airport.
Meanwhile, Frankfurt airport operator Fraport said it was not interested in acquiring a stake in BAA.
A Fraport spokesman said: "There is no question of taking part in a consortium."
In 2003 Grupo Ferrovial acquired the UK engineering firm Amey.
Airport sector analyst Keith Bowman of Hargreaves Lansdown said Ferrovial's offer had come as "a complete surprise".
"There is likely to be much deliberation over such a bid and it is almost unthinkable that the UK government would not become involved," he said.
"What the potential bid does again reaffirm is the significance of so called 'strategic assets' as with the current bids for the London Stock Exchange and P&O ports."
In addition to its UK operations, BAA has a stake in 13 overseas airports, including operations in the US, Australia, Italy and Hungary.
It is the largest airport operator in Europe.
BAA shares closed up 97.5 pence at 752.5p.