Shares in airports operator BAA rocketed more than 20 per cent yesterday after a Spanish infrastructure company signalled a possible takeover move.
Grupo Ferrovial, which owns construction company Amey, said no approach had yet been made, but added that any offer was likely to be as part of a consortium.
A spokeswoman for Macquarie Airports, which has a stake in Birmingham Airport, would not say whether it was interested.
"We do not comment on market rumours," she said.
Ferrovial already has a stake in Bristol Airport in a joint venture with Macquarie and complete ownership of Belfast City airport.
BAA shares leapt to a new high of 801 pence on the news, which some analysts said overvalued the business.
The company owns and operates Heathrow, Gatwick, Stansted, Edinburgh, Glasgow, Aberdeen and Southampton airports.
Ferrovial issued its surprise statement in response to recent movements in the BAA share price.
It said: "Ferrovial's considerations are at a preliminary stage and there is no certainty that any offer will be made. No approach has yet been made to the board of BAA."
The company is one of Europe's leading construction, services and infrastructure groups, with more than 68,000 employees and a presence in 40 countries. It has annual revenues of more than seven billion euros (£4.8 billion).
BAA declined to comment on Ferrovial's announcement.
However, the group advised shareholders not to take any action, adding that it was confident about the future of the business."
Keith Bowman, equity analyst at Hargreaves Lansdown Stockbrokers, said the move had come as "a complete surprise".
He added: "There is likely to be much deliberation over such a bid and it is almost unthinkable that the Government would not become involved.
"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."
Frank Skodzik at WestLB said: "According to my valuation, the current market capitalisation of BAA is already higher than what's fair value. If they pay over current market cap, then they are already paying a premium."
The Government's "golden share" in BAA was ruled illegal under EU law in May 2003.
The "golden share" system was devised to give the Government a continuing stake even after nationalised industries were privatised, ensuring that undesirable takeovers deemed against the national interest could be vetoed.
Another key provision of the system prevented any one shareholder from holding more than 15 per cent of the voting shares.
The ruling effectively removed the Government's power to protect BAA from hostile takeover bids, ending the special protection it had enjoyed since its flotation in 1987.
Ferrovial has traditionally made most of its profits from Spain's booming building industry, but more recently
has expanded overseas and into infrastructure services to offset cyclical construction operations.
Last August, the group bought airport handling firm Swissport and said it planned to bid for contracts to run Spanish airport services.
Airport services are enjoying a boom as low-cost airlines fuel a rise in passenger traffic, and consolidation is picking up as state-owned operators are privatised. Shares closed up 971/2p at 7521/2p.