MILAN (Reuters) - Italian utility Enel (>> Enel S.p.A.) has presented an informal offer to buy a majority stake in fibre network company Metroweb as part of its plans to roll out a high-speed Internet network, a source familiar with the matter said on Saturday.

The offer would see Enel acquire the 54 percent stake in Metroweb currently owned by infrastructure fund F2i for around 400 million euros (£316.1 million), valuing the whole of Metroweb at 776 million euros, the source said.

The preliminary, informal bid was presented to state-owned Cassa Depositi e Prestiti (CDP), which owns the rest of Metroweb and a minority stake in F2i, the source added. Effectively, any friendly bid for control of Metroweb would need CDP's consent.

Enel and CDP declined to comment. F2i said in a statement it had not received an offer for its stake in Metroweb, adding it would assess any proposal in the interest of its own investors and of the fibre network company.

Rolling out a national ultra-fast broadband network is a key plank in the reformist agenda of Prime Minister Matteo Renzi who has enlisted state-controlled Enel's help to achieve the goal.

Taking a majority stake in Metroweb would boost Enel's ambitions because of its fibre assets and skills.

Italy's biggest utility earlier this year set up Enel Open Fiber (EOF) to develop its fibre ambitions and plans to invest 2.5 billion euros to bring ultra fast broadband into peoples' homes using its existing power network.

Under the informal proposal, Enel Open Fiber and CDP would jointly manage Metroweb, the source said.

However, phone group Telecom Italia (>> Telecom Italia SpA) is also interested in taking full control of Metroweb to boost its own plans to expand fast Internet coverage in Italy.

The source said Telecom Italia was expected to present its own, rival offer for Metroweb as soon as Monday. A second source said Telecom Italia would make its offer early next week.

Enel's board meets on Monday to approve first-quarter results and should also discuss the offer for Metroweb, although no statement on the matter is expected. Telecom Italia's board meets on May 13.

The former state telecoms monopoly is in parallel talks with F2i and CDP to buy Metroweb for cash and in exchange for a stake in its international wholesale unit Sparkle, which may give its offer an edge over Enel's, sources have said.

CDP, which controls Italy's gas and power grid companies Snam (>> Snam SpA) and Terna (>> Terna - Rete Elettrica Nazionale SpA), is keen to tighten its grip on strategic networks in the country.

Sparkle is deemed strategic because its submarine network transmits information between countries in Europe, the Mediterranean and the Americas.

If CDP were to own a slice of Sparkle this would help the government prevent it from falling into foreign hands, especially given the growing influence of French media group Vivendi (>> Vivendi) as the top investor in Telecom Italia.

The decision on the fate of Metroweb is politically sensitive and the government is likely to wait until municipal elections in early June before choosing between Enel's and Telecom's offers, another source said.

(Additional reporting by Stephen Jewkes and Stefano Rebaudo; Editing by Mark Bendeich and Toby Chopra)

By Silvia Aloisi