The company says the tape contains names, account information, payment histories and social security numbers for residential mortgage customers.

The tape went missing in November while being transported by DHL from the company's Chicago data processing centre to an Experian credit bureau facility in Texas. A package containing the computer tape was picked up but never arrives at the Experian site.

Thomas Goldstein, chief executive of ABN Amro Mortgage Group, says the company has begun notifying customers and informing them of steps they can take to protect themselves.

Goldstein says ABN Amro has also suspended physical tape transfers to credit bureau companies following the loss of the tape, and is transmitting that customer data from its Chicago data processing centre "exclusively by secure and encrypted electronic means".

He says there is no reason to believe that the missing data has been misused but the company has implemented measures to check for any unusual activity. The company has also arranged for its residential mortgage customers to enroll in a credit monitoring service for 90 days at no cost to them.

ABN Amro has begun an internal investigation into the matter and has notified the appropriate regulatory agencies. It has also asked DHL to conduct an investigation.

Earlier this year CitiFinancial, the consumer finance subsidiary of Citigroup, reported that unencrypted tapes containing the personal financial data of 3.9 milllion customers had been lost by the bank's third party courier UPS while in transit to a credit bureau.

In December last year Bank of America lost computer tapes containing charge card account information for 1.2 million government card holders during transfer to a backup data centre.

At the time, Bank of America's failure to encrypt the data in advance of transfer was widely criticised by security professionals. This lesson appears to have been lost on ABN Amro.