ROME—Troubled Italian lender Banca Monte dei Paschi di Siena SpA is set to launch its seventh share sale in 14 years, after securing the backing of a group of investment banks that have agreed to buy any unsold shares. The bank hopes to raise €2.5 billion, equivalent to $2.4 billion, to pay for restructuring costs and shore up its capital base under terms agreed with European authorities earlier this year.

---