BAA operates seven UK airports

Macquarie has agreed to become a joint financial adviser to Grupo Ferrovial's bidding consortium, alongside existing adviser Citigroup.

In return, Ferrovial is giving Macquarie the option of buying its stakes in Sydney and Bristol airports.

BAA rejected a �8.75bn takeover proposal earlier this month from the Ferrovial consortium.

If the takeover bid eventually succeeds, Macquarie would have the option to buy Ferrovial's 20.9% percent stake in Sydney Airport for about �408m ($711mn) and its 50% stake in Bristol Airport for about �106m, the Spanish group said.

Old partners

The bidding group includes Canadian fund manager Caisse de Depot et Placement du Quebec and the private equity investment arm of the Singapore government.

It brings Macquarie on side, and I think therefore increases the likelihood that we will see an improved offer from the Ferrovial consortium Damian Brewer, analyst See BAA's share price

The UK's takeover watchdog said last week that the Ferrovial consortium must make a firm offer for BAA by 24 April, or walk away from the bid arena.

Macquarie and Ferrovial have worked together in the past, jointly paying �198m to buy Bristol Airport in 2001, as well as teaming up on a toll road project in Chicago and a 253km highway running the full length of the US state of Indiana.

Analysts said the deal effectively keeps one potential competitor at bay and strengthens Ferrovial's position.

"It brings Macquarie on side, and I think therefore increases the likelihood that we will see an improved offer from the Ferrovial consortium," said JP Morgan analyst Damian Brewer.