Tuscany's bailed-out Monte dei Paschi had unexpectedly launched a 13-billion-euro all-share takeover proposal for Mediobanca.

The board of Mediobanca said Banca Monte dei Paschi di Siena’s offer would destroy value for shareholders of both banks.

Monte dei Paschi, Italy’s oldest bank, launched a 13.3 billion-euro ($13.9 billion) takeover bid last week for larger Milan-based peer Mediobanca that aims to reshape the Italian banking sector. Monte dei Paschi said that the tie-up would generate 700 million euros ($733 million) a year in pretax synergies, and deliver significant profits.