Intesa Sanpaolo has been ordered by Italy's competition authority to stop the migration of 2.4 million customers to its new app-only offshoot unless it gets express consent from the users.
The Italian lender launched its cloud-based, app-only Isybank this summer as part of a major digital transformation programme.
In October, it migrated around 300,000 customers to the unit, sending them messages with the option to opt out.
The Autorità Garante Della Concorrenza e del Mercato (AGCM) says it has received 5000 complaints about the switch.
According to the AGCM, messages about the move were sent to customers in the archive section of the Intesa Sanpaolo App without any push notifications or pop-ups.
Customers were unhappy about this, as well as the fact that the messages were sent during the August holidays and that the new accounts have "different economic conditions".
In addition, users complained about the loss of some services post-move, such as the ability to create virtual cards for online purchases.
Therefore, the AGCM has told Intesa that it cannot migrate the rest of the 2.4 million customers to Isybank without their express consent, ensuring their right to keep their current account under the same conditions.