HSBC's virtual bank first direct has mistakenly sent customers text messages containing the transaction details of other people.
First direct's text banking customers are sent daily messages detailing their balances and transactions from the previous day.
But this morning, texts sent to a "small number" of customers contained transaction details related to other accounts.
A bank spokeswoman, who was herself the recipient of a rogue message, says the texts contain details on transaction amounts and, in some cases, where the purchase was made. She says there is no security risk because no account details are involved.
The problem is suspected to be the result of work carried out overnight to increase the capacity of the "pipe" used by first direct to deliver its text messages.