MILAN/ROME (Reuters) - Italian airports and motorway operator Atlantia (>> Atlantia) said on Friday it would call a shareholder meeting to vote on changes to its offer for Spanish rival Abertis (>> Abertis Infraestructuras), as it seeks to strengthen its hand in a bid battle.

Atlantia said it proposed to shorten the lock-up period during which Abertis shareholders would not be able to sell new Atlantia stock they would receive as part of its bid.

The Italian firm also said its board would decide whether to modify the price, structure and other conditions of its offer once a rival bid was approved by regulators.

Atlantia is competing against Hochtief (>> Hochtief), the German arm of Spanish builder ACS (>> ACS Actividades de Construccion y Servicios), in the battle for Abertis.

Hochtief is currently in the lead, having submitted a higher bid on Oct. 18 that values Abertis at around 17.1 billion euros.

Atlantia's cash-and-share bid values Abertis at about 15.6 billion euros and has a 10.1 percent minimum acceptance threshold for the share component.

On Friday, the Italian group said its board could decide to waive that minimum acceptance threshold and other conditions.

"Atlantia is open to turning its offer into an all-cash bid," a financial source told Reuters.

An extraordinary shareholders' meeting called for Feb. 21 will vote on the postponement to Nov. 30 of a deadline to carry out a cash call linked to the bid, Atlantia said in a statement.

The meeting will also vote on shortening the lock-up period for the share part of its offer to 90 days from a period of around a year previously.

(Reporting by Francesca Landini and Stefano Bernabei in Rome; Editing by Mark Potter)