MILAN (Reuters) -Monte dei Paschi di Siena said on Wednesday the European Central Bank had approved its proposed acquisition of rival Mediobanca, including under a scenario in which it gains a stake of less than 50% in the rival.

The ECB approval, which Reuters first reported on Monday, paves the way for Monte dei Paschi (MPS) to formally launch the offer next month, inviting Mediobanca shareholders to tender their shares.

MPS can count on the support of a group of significant Mediobanca shareholders - the billionaire Del Vecchio and Caltagirone families, who became investors in MPS last November when the Treasury last sold a chunk of the bailed-out bank.

The November placement is now being investigated by Milan prosecutors, and details of the probe are not known.

The ECB has set conditions MPS should meet following the acquisition, such as filing within six months of completing the deal an integration plan with details, among other things, of strategies for funding and capital, IT and cybersecurity risks, and remuneration policies to retain Mediobanca bankers.

The deal brings together MPS' commercial franchise with Mediobanca's investment banking and wealth management businesses. The two lenders already partner in consumer finance.

Mediobanca has strived to fend off MPS, but this month it had to postpone a shareholder vote on an alternative project, fearing it may lack sufficient support.

If it acquires a stake of less than 50%, MPS must submit within three months a report on whether it can exercise de facto control of Mediobanca or, alternatively, its strategy concerning the stake in the absence of actual control.

MPS was rescued by the government in 2017 and is still 11.7% owned by Rome. The hostile bid for Mediobanca it unveiled in January is one of several unsolicited takeover offers that are reshaping Italian finance as banks turn to consolidation to feed profits with interest rates falling.

(Reporting by Valentina Za; Editing by Tomasz Janowski and Jan Harvey)