The potential shareholders must present binding offers before Oct. 8 to take a stake of up to 21 % in the operator in blocks of between 5 and 11 percent. Qualification documents were to be submitted on Sept. 23 and the list of interest of bidders is expected to be released today.
Secretary of State for Infrastructure Rafael Catala said the government was working on preparing the prospectus for the sale.
The entry of investors into Aena would take place in two stages. The first phase will involve the sale of a 21% stake to selected investors chosen via a tender process.
The second phase will involve a public offering of 28% of Aena's share capital that will be open to minority investors.
Spanish newspapers said Spanish groups Corporacion Alba and Ferrovial, and investment funds such as KKR, CVC and Macquarie were among potential bidders. However, sources said that several bidders would only participate if it gets a commitment that it would be able to have a say on the firm's decisions in the future.
The fact that the state will retain control of the group meant it is less attractive for infrastructure operators.
According to Manuel Vargas, Aena's president, the firm is worth about €16 billion. Aena carries a debt of more than €11 billion.
Aena is the world's leading airport manager, moving 187.4 million passengers in 2013. The company runs 46 airports and two heliports in Spain and another 15 airports abroad, most in Mexico.
The four stand-out airports in AENA are Madrid, Mallorca, Malaga and Barcelona.