Star Capital Partners is thought to have been working on a bid for the airport operator, which has also attracted the interest of Spanish contractor Ferrovial, for several weeks.

Another potential bidder has joined the race to snap up BAA, the £8.4bn owner of the main British airports.

Star Capital Partners, the private equity firm, is thought to have been working on a potential bid for BAA for several weeks. The bid preparations are at an early stage and Star Capital is yet to make an approach to BAA.

Star Capital's interest in BAA comes after news broke on Wednesday that Grupo Ferrovial, the Spanish infrastructure company, was considering a bid for the airport operator. Ferrovial has also not yet approached BAA.

Ferrovial's admission, triggered by a takeover panel inquiry, could flush out other prospective bidders for BAA, including Vinci, the French group, and the acquisitive Australian bank Macquarie.

Macquarie, which part-owns Bristol airport with Ferrovial, has refused to rule itself out of a BAA bid, with or without Ferrovial. A spokesperson would say only that the company had not been approached by Ferrovial to join a bid consortium.