Lloyds TSB has written to 4,000 of its customers to warn them that a hand finishing error may have led to credit card details being sent to the wrong people.
The mailshot was for a promotion to sell insurance against credit card fraud to a section of Lloyds TSBs existing customers.
A spokeswoman for Lloyds TSB said that 40 customers had been affected by the collating blunder, and that the risk of misuse was "very low".
She said: "It is not company policy to reveal the name of the outsourcing company. We expect those affected to stay in the double figures."
The spokeswoman admitted that human error was probably to blame.
She added: "It appears some letters and application forms were mixed up, so some customers got the wrong card details with their letter."
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