The complaints prompted Italy's central bank as well as the antitrust authority to look into the way the lender was shifting the clients to the app-based unit.

In a note sent to customers on Monday and seen by Reuters, the bank wrote that anyone wanting to remain with Intesa Sanpaolo had until the end of February to say so. A previous communication issued in July had set the deadline at Sept. 30.

Intesa San Paolo was not available for comment.

Letizia Giorgianni, from the Brothers of Italy party led by Prime Minister Giorgia Meloni, welcomed the move, saying Intesa's customers have the right to choose by whom and how to have their savings managed.

Isybank, a cloud-based, low-cost mobile bank, is a key plank of Intesa CEO Carlo Messina's long-term strategy to withstand competition from fintech by cutting costs and focusing the efforts of expensive branch staff on value-added wealth management and non-life insurance services.

The competition watchdog took aim at how Intesa informed account holders of the move, adding it had received more than 2,000 complaints.

The message was "ambiguous and sent in a way that is not consistent with the importance of the matter at stake," the regulator said earlier this month.

(Reporting by Giuseppe Fonte; Editing by Keith Weir)