"The group's steering committee has given a negative opinion to Intesa Sanpaolo's proposal," the shareholders, which call themselves 'Patto dei Mille', said in a statement.

"The swap offer significantly underestimates the intrinsic value of UBI's stock and does not adequately consider its earnings outlook."

Intesa last week announced an all-paper offer to buy smaller peer UBI Banca as it seeks to create the euro zone's seventh-largest bank by assets.

The group of wealthy families and business owners from the northern town of Bergamo said the offer does not protect shareholders' interest.

(Reporting by Andrea Mandalà; editing by Francesca Landini)