MADRID (Reuters) - Atlantia and ACS must seek consent from the Spanish government to complete their joint bid for motorway firm Abertis, Public Works minister Inigo de la Serna said on Thursday.

An agreement between Spain's ACS and Italy's Atlantia to make a joint 18 billion euro bid for Abertis will create a new company which will hold the Spanish toll-road operator's assets and will be jointly run.

The deal was partly designed to ease Spain's concerns over strategic assets falling to foreign business interests after Atlantia launched a full takeover offer, and it has been closely scrutinised by the government.

Abertis holds various concessions for important Spanish highways as well as stakes in Spanish telecoms company Cellnex and a strategic satellite business.

The comments by De la Serna show Madrid, which received the deal favourably, was treading carefully until the full details of the merger are finalised.

"A change in shareholders will probably need to seek a new authorisation from the government to manage for example, the highways," De la Serna told public broadcaster RTVE in an interview.

The full details of the deal are still to be finalised and a new bid by ACS's German arm Hochtief may require another round of authorisations from regulatory bodies in Europe and Spain.

(Reporting by Rodrigo de Miguel; Writing by Paul Day; Editing by Julien Toyer and Elaine Hardcastle)