A security lapse by Nationwide Building Society may have put 11 million customers at risk of identity theft.
The Financial Services Authority is investigating the incident after an employee's laptop was stolen, containing sensitive customer account information.
The computer was taken during a domestic burglary in August, but details are only now coming to light.
Nationwide confirmed the laptop held customer information, but insisted this did not include pin numbers, passwords or information about financial transactions.
The building society is writing to all of its 11 million customers about the importance of security.
Nationwide's chief executive Philip Williamson explained why the bank did not inform customers earlier.
He told BBC Radio 4's Today programme: "The reason we didn't let customers know about this incident earlier is we didn't wish to create undue anxiety.
"This was a view shared with the police, the Financial Services Agency and the Information Commissioner."
Mr Williamson added: "Our analysis of the situation at the time led us to the conclusion that no customer would lose out as a result of the theft.
"There were no PIN numbers, no passwords, no account balance information or memorable information relating to any customers and the stolen laptop was security protected. On the strict advice of the police, I am unable to disclose details around the theft."
