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

Italian bank Mediobanca on Tuesday rejected a takeover offer by state-backed rival Monte dei Paschi di Siena , saying a tie-up would be detrimental to its shareholders because it lacked any strategic and financial rationale.

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.