UPDATE 2-Loan data stolen, National Bank of Canada says

(Adds details, background on data breaches)

TORONTO, Sept 23 (Reuters) - National Bank of Canada (NA.TO) said on Tuesday that a portable computer containing data about mortgage customers was stolen late last week from the bank's head office in Montreal, but it said the risk of fraud or identity theft was "minimal."

The bank is not saying how many clients were affected, but spokesman Denis Dube said names, addresses and bank reference numbers for "a high percentage" of its mortgage customers were in the database.

However, the computer contained no further personal data such as social insurance numbers or credit information, Dube said.

"That's why we say the impact will be very minimal," he said.

Police are investigating the "unfortunate" incident and the computer has not been recovered, Dube said.

National Bank, the sixth-largest bank in Canada, said it would protect clients by informing them quickly about the event, asking them to report unauthorized transactions in accounts, and compensating clients for related damage if necessary.

Anne-Marie Hayden, a spokeswoman for the federal privacy commissioner's office, said that National Bank had informed the office about the breach.

Other banks have had data breaches in the recent past, and complaints about financial institutions have topped complaints about other sectors covered by Canada's personal information protection laws for several years in a row.

In January 2007, Canadian Imperial Bank of Commerce (CM.TO) said a backup computer file containing personal information on nearly half a million Talvest mutual fund customers disappeared.

At the time, CIBC said the missing Talvest file may have included not only names and signatures, but birthdates, bank account information or social insurance numbers.

The privacy commissioner launched an investigation in the Talvest case. (Reporting by Lynne Olver; editing by Rob Wilson)