HSBC Bermuda yesterday apologised after it e-mailed personal information on customers to other account holders.

The e-mails contained names, e-mail addresses, countries of residence, the name of the customers’ relationship manager and HSBC customer identification numbers.

Now the bank has launched an investigation to find out how the personal details — attached to an e-mail about HSBC world selection portfolios — were released to third parties.

A spokeswoman for the bank declined to say how many customers were affected by the mistake.

She said: “An e-mail was sent to a small number of our retail banking customers which unfortunately included an attachment containing personal information of some of HSBC Bermuda’s customers.”

She added the identification numbers were used internally at the bank to identify account holders and that no other information was released.

The spokeswoman said: “All impacted customers have been contacted, either by telephone and/or e-mail and provided instructions.”

The e-mail to customers said the information “on its own” could not be used to access accounts, but warned clients to be “extra vigilant in the review of your accounts for the foreseeable future”.

The e-mail added that customers who had received information on other account holders had been asked to delete the attachment.

The spokeswoman said: “No further action is required from them and customers can confidently continue their banking transactions.

“As always, we encourage customers to remain vigilant of their accounts and report any other unusual activity through our usual channels.”

She added: “The HSBC Group takes the security of its customer information extremely seriously and constantly reviews systems and security.

“We are currently reviewing this matter so that the root cause can be identified to prevent any future incidents.”