MADRID/MILAN (Reuters) - Spain's market watchdog approved Italian infrastructure group Atlantia's (>> Atlantia) proposed takeover of Spanish rival Abertis (>> Abertis Infraestructuras) on Monday, clearing one of the hurdles to the creation of the world's biggest toll-roads operator.

Approval comes nearly five months after Atlantia announced it was offering 16.3 billion euros (14.55 billion pounds) in cash and shares to buy Abertis.

Spanish regulator CNMV said in a market filing the offer was conditional on shareholders representing at least 10.1 percent of Abertis share capital accepting payment in Atlantia shares.

In addition the offer must be accepted by shareholders representing at least 50 percent plus one share of the Spanish company.

Atlantia is offering to pay 16.5 euros for each Abertis share, a level that CNMV said was equitable.

Abertis shareholders will have 15 calendar days to accept the bid starting from the first trading day following the formal publication of the offer's prospectus, the watchdog said.

One source with knowledge of the matter said this was expected to happen in the next few days.

Though the offer was described as "friendly" by Atlantia, Abertis's shareholders have not expressed a view on the bid and potential counter-bids could still emerge.

The board of the Spanish company must publish its assessment of the bid within 10 days after the start of the acceptance period, Atlantia said.

The top shareholder in Abertis is Criteria Caixa, the financial arm of a politically connected and powerful banking foundation that controls Catalonia's largest lender Caixabank (>> CaixaBank).

Several potential rivals for acquiring Abertis have been named in media reports in recent months, but only Spanish builder ACS (>> ACS Actividades de Construccion y Servicios), which is headed by Real Madrid soccer club Chairman Florentino Perez, has said it is looking at making a possible counter-offer.

ACS is expected to decide this month whether to make an offer.

In the prospectus published on the website of Spain's CNMV, the Italian group said it was leaving the door open to changing the bid to an all-cash offer.

Atlantia, controlled by Italy's Benetton family, also said in the document it would sell a portion of Abertis's 34 percent shareholding in telecom masts company Cellnex (>> Cellnex Telecom) to avoid having to make a full bid for Cellnex.

It also repeated it was ready to sell Abertis's satellites business Hispasat if requested by the Spanish government.

(writing by Isla Binnie, Francesca Landini; Editing by Greg Mahlich)

By Paul Day and Francesca Landini