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."
Have your say in the Printweek Poll
Related stories
Latest comments
"Thank you for the opportunity to comment on this Jo, and PrintWeek!
Please feel free to get in touch with the Howden Print Team to arrange your own Free of Charge Cyber Micro-Penetration Test to help..."
"I never quite understand the statements such as "achieved such a positive outcome for this well-established business".
The established business unfortunately failed and no longer exists, a..."
"Genuinely sorry to read this."
Up next...

Commitment, presence and energy will be much missed
Tributes paid to Lascelle Barrow

Around 300 roles may be affected
International Paper to close five UK packaging sites

Asset ownership delayed process
Reflections to be liquidated

'Start of a new era'