TORONTO -- CIBC's direct banking brand Simplii Financial has joined BMO in offering free credit monitoring for tens of thousands of customers whose personal and financial information may have been accessed by "fraudsters."

A CIBC spokesman says the bank is replacing affected clients' bank cards and taking additional steps to monitor and protect the approximately 40,000 Simplii clients who may have been impacted by the breach.

BMO said yesterday it was offering free credit monitoring, and will additionally block online and mobile access to accounts of those affected, which the bank believes is fewer than 50,000.

Both banks are also vowing to return 100 per cent of any money lost by impacted customers through the incident.

BMO said it learned of the potential breach on Sunday when it was contacted by individuals claiming to be in possession of its clients sensitive information and threatening to make it public.

CIBC learned of the potential issue on the same day, and says it continues to reach out to those that have been impacted to offer support.