LONDON/MILAN/MADRID (Reuters) - Italian toll road operator Atlantia (>> Atlantia SpA) is planning to submit a takeover bid for Spanish rival Abertis (>> Abertis Infraestructuras S.A.) within days, two sources familiar with the situation said on Tuesday, hoping to bridge differences over price that have held up negotiations in the past few weeks.

A merger between Atlantia and Abertis would create Europe's biggest toll road group, with a combined market value of more than 36 billion euros (£30.3 billion).

It would also speed up Atlantia's plans to diversify away from its home market, with the combined group generating around 60 percent of its core profit outside Italy.

Atlantia, controlled by the Benetton family, confirmed last month it was considering a deal with Abertis providing it was friendly and created shareholder value. "It's a matter of days. The offer could come towards the end of this week or early next week," one of the sources said, adding that the two parties were keen to reach a friendly agreement. Representatives at Atlantia were not immediately available for comment, while Abertis declined to comment.

The Rome-base group has yet to hammer out the final terms of the deal, the sources said, and plug differences over Abertis' valuation.

Two of the sources said Atlantia had initially approached Abertis with a 16 euro-a-share bid but Abertis's main shareholder, La Caixa, had asked for 17 euros per share, valuing the Barcelona-based business at around 16.8 billion euros, slightly above its market value of 16.6 billion euros.

A third source said the Spaniards wanted a valuation above 16 euros per share.

"The final price is likely to be somewhere in the middle. I don't think the Benettons will pull out due to a relatively minor valuation difference," one of the sources said.

Abertis shares were trading at 16.28 euros at 1304 GMT.

The sources said Atlantia has secured a financing package worth around 11 billion euros to back a cash and share bid.

Reuters previously reported that Atlantia's advisers Credit Suisse (>> Credit Suisse Group AG) and Mediobanca (>> Mediobanca Group) and Abertis' adviser Citi (>> Citigroup Inc) had committed to provide financing for the deal.

The pool of lending banks will also include Italian lenders UniCredit (>> UniCredit SpA) and Intesa Sanpaolo (>> Intesa Sanpaolo SpA) and France's BNP Paribas (>> BNP Paribas) and Societe Generale (>> Société Générale) among others.

Atlantia will hold a board meeting on Friday to discuss its first-quarter results and may try to formalise its bid for Abertis, the sources said, although a final decision had yet to be taken.

Atlantia, which operates Rome's two airports and around 5,000 km of toll motorways, has long been trying to lure its Spanish rival to the negotiating table, the sources said.

But Abertis, considered one of Catalonia's "crown jewels", has only recently started contemplating the possibility of a sale to enable the business to cope with domestic challenges including a series of concessions that will soon expire, the sources said.

(Additional reporting by Francesca Landini and Stephen Jewkes in Milan. Editing by Jane Merriman)

By Pamela Barbaglia, Paola Arosio and Robert Hetz