MILAN (Reuters) - Bailed-out lender Monte dei Paschi di Siena (MPS) said on Friday it was launching a 13.3 billion euro ($13.9 billion) all-share buyout offer for Mediobanca, in the latest surprise twist of a complex Italian banking saga.

Monte dei Paschi (MPS), which for years was the problem child of Italian banking until a 2017 bailout, is offering 23 of its own shares for every 10 Mediobanca shares tendered, equivalent to a 5% premium versus Thursday's closing price.

Mediobanca has a market value of 12.7 billion euros, compared with Monte dei Paschi's capitalisation of 8.8 billion.

The buyout offer comes after Italy's drive to re-privatise the Tuscan bank brought onboard as shareholders in November Delfin, the holding company of late billionaire Leonardo Del Vecchio, and fellow tycoon Francesco Gaetano Caltagirone.

Delfin is the biggest shareholder in Mediobanca with a 19.8% stake while Caltagirone owns 7.8%.

Delfin nearly tripled its initial MPS holding to 9.8% in January.

Mediobanca, Caltagirone and Delfin are all large shareholders in insurer Generali, accounting for almost a third of its capital base.

Caltagirone, who had also initially bought 3.5% of Monte dei Paschi, increased it to 5% in November.

MPS has successfully restructured in recent years under CEO Luigi Lovaglio, a veteran UniCredit executive.

Italy, as it gradually reduced its stake to 11.7% from the initial 68%, has been seeking a partner for MPS, which like other mid-sized banks faces long-term challenges due to the need for hefty technology investments and the threat from non-bank players.

Since UniCredit walked away from a deal back in 2021, the Treasury has been working on a potential tie-up with Banco BPM, which became a shareholder in MPS alongside Delfin and Caltagirone in November.

That plan was derailed by UniCredit's decision late last year to launch a buyout offer for Banco BPM, as CEO Andrea Orcel said his bank could not afford to be sidelined in the consolidation process.

($1 = 0.9568 euros)