MADRID (Reuters) - An agreement between Spain's ACS and Italy's Atlantia to make a joint 18-billion-euro ($22.2 billion) bid for Abertis will not lead to a break-up of the Spanish toll-road operator's assets, ACS's chairman said on Thursday.

Spanish builder ACS and Atlantia agreed to jointly bid for Abertis on Wednesday, ending a five-month bidding battle and easing political concerns.

Spain's government had worried that an Italian takeover would have left important highways and a strategic satellite business under foreign ownership. However, the proposed joint bid will be made by ACS's German arm Hochtief, securing Spanish influence.

Spanish Energy Minister Alvaro Nadal welcomed the agreement as ACS and Atlantia would avoid a costly takeover battle.

"It seems to me a good outcome that there is a business project for a company that is so important for Spain," Nadal told radio station Onda Cero on Thursday.

The new company that emerges from the joint bid will be Spanish, ACS Chairman Florentino Perez told a news conference in Madrid.

Some newspapers and analysts had speculated that Atlantia and ACS could break up the Abertis business in future, but Perez denied that was the intention.

"We've not reached this agreement to break up Abertis and share out its assets. Full stop. This is a long-term project," Perez said.

However, analysts were sceptical about the long-term future of the new holding company.

"The deal creates ties between the groups. However, experience suggests that in the long term joint ventures between large groups can be difficult," UBS said in a note to clients.

Under the proposed deal, Atlantia will own 50 percent plus one share in the entity which will ultimately own Abertis, plus an additional, indirect stake through a related purchase of around 24.1 percent in Hochtief.

Atlantia Chief Executive Giovanni Castellucci said on Thursday the deal was a "big opportunity."

The chairman of the resulting company will be appointed by ACS and its German affiliate Hochtief, while Atlantia will name its CEO, ACS's Perez said.

Neither Perez nor Castellucci said whether Atlantia would exercise its option to buy from Abertis a stake of 29.9 percent or 34 percent in Spanish telecoms group Cellnex.

Perez, who is also chairman of Real Madrid soccer club, said that toll-road operator Sanef, a French unit of Abertis, would remain an important asset and would be jointly managed.

Atlantia and ACS said on Wednesday that Hochtief would first launch an 18.2 billion euro cash bid for Abertis, before transferring it to a vehicle which will be 30 percent owned by ACS and almost 20 percent by Hochtief.

A shareholders' pact and a long-term contract among the three groups will set governance rules.

A group of banks including Credit Suisse, BNP Paribas, JPMorgan (>> JP Morgan Chase & Company) and Italy's UniCredit have arranged a 14 billion euros financing package for the deal, two sources said.

On Thurday, ACS and Abertis's shares were almost unchanged.

Hochtief and Atlantia fell by around 3 percent and 2 percent respectively by 1350 GMT.

($1 = 0.8093 euros)

(Reporting by Jesús Aguado and Jose Elias Rodriguez; Writing by Paul Day; Editing by Angus Berwick and Adrian Croft)

By Jesús Aguado and Jose Elías Rodríguez