Hundreds of National Bank customers may have had their personal contact information and date of birth exposed online to other customers, the bank confirmed to CBC Toronto.

The problem lasted for several days and may have affected "close to 400" customers across Canada, according to a statement from National Bank.

"We were notified earlier this week of an issue related to an electronic form used on our website. The issue was resolved immediately," said National Bank senior director Jean-François Cadieux. "No address, Social Insurance Number or any banking information have been disclosed."

A customer filling out the online form could have viewed the information of the customer who previously filled it out. (CBC)

Essentially, a customer filling out the online form could have viewed the information of the customer who previously filled out the form, Cadieux said.

CBC Toronto was alerted to the problem by a bank customer who said he was contacted by another customer who viewed his contact information online.

He was told it happened when the other customer attempted to book an appointment with the bank.

In order to do so, the customer had to enter their contact information, but the fields were already full, he said.

National Bank says the problem was due to human error. (CBC)

The customer told CBC Toronto he then verified this by returning to the website and attempting to book an appointment. When he did so, the fields were already full with other customers' information.

The customer sent CBC Toronto several screenshots showing contact information and the date of birth for several people from Toronto, Montreal and the Vancouver area. CBC Toronto confirmed the information was accurate.

According to National Bank, the problem was the result of human error. Potentially impacted customers are being contacted by the bank to offer them free credit monitoring surveillance and to make sure they stay vigilant.