MILAN (Reuters) -Poste Italiane has agreed to buy the bulk of Vivendi's stake in Telecom Italia to replace the French group as the main investor in the phone group with a 24.8% holding, Poste said on Saturday.

Confirming what sources had earlier told Reuters, Poste said it was buying a 15% stake in TIM from Vivendi, which had recently reduced its holding to 18.4%.

Poste said it would use available cash to fund the 684 million euro ($741 million) investment, buying shares in TIM at 0.2975 euros each.

That compares with a closing price for TIM on Friday of 0.3126, equivalent to a 4.8% discount.

Poste said it would remain below the 25% threshold above which, under Italian law, it would have to make an offer for the rest of TIM.

It added it would act as a long-term shareholder in TIM following the "strategic investment", looking for potential benefits from cooperation between the two companies and supporting sector consolidation.

State-controlled Poste, whose businesses include energy, payments and phone services as well as mail and parcels, first became an investor in TIM in February with a 9.8% stake, replacing state lender CDP.

TIM is Italy's former phone monopoly and a business of strategic national importance.

After spinning off its fixed-line grid last year, TIM could now become involved in the consolidation expected to happen in a sector where profit margins are seen as too thin to finance the required investments.

For Rome to preserve a meaningful presence in TIM if it ties up with another telecoms firm, Poste first needed to boost its stake.

Reuters was first to report that Poste was ready to increase its holding. Sources said earlier this week that Poste was in talks with Vivendi.

A senior Treasury official also said this week that Poste had "the right credentials" to be a leading investor in TIM because it could be both a financial and an industrial partner.

($1 = 0.9236 euros)

(Additional reporting and writing by Valentina Za.Editing by Mark Potter)