Unipol said it had hired Equita SIM and Mediobanca to launch a reverse accelerated bookbuilding process and buy Popolare di Sondrio shares at a price no higher than 5.1 euros each.

Shares in Popolare di Sondrio ended the session at 4.902 euros, meaning Unipol is offering a 4% premium to that closing price.

While the offer is conditional on Unipol acquiring 10.2% of the bank, the group said it reserved the right to accept a smaller stake.

Unipol already owns 9.5% of Popolare di Sondrio through its UnipolSAI unit, which Italy's second largest insurer.

It has gained European Central Bank authorisation to cross the 10% threshold and get to 19.99%, it added.

"The stake increase will allow Unipol to strengthen the ... insurance partnership with Popolare di Sondrio ... render the bank's shareholder base more stable to better pursue its ... goals and support its evolution in line with the best market practices," it said.

Unipol is also the top shareholder in mid-sized bank BPER Banca.

Long seen as an attractive takeover target, Popolare di Sondrio has been the last large cooperative bank in Italy to ditch the cooperative status that shielded it from potential takeovers, finally complying with a government reform after losing a court battle to oppose the change.

Unipol's stakebuilding in Popolare di Sondrio has long stoked speculation of a potential eventual tie-up between Popolare di Sondrio and BPER.

(Reporting by Valentina Za; Editing by Emelia Sithole-Matarise)