Wokingham Borough Council wrote to those affected on Tuesday after their names and addresses from the electoral register were accidentally disclosed to third parties for marketing purposes – despite their decision to opt out.
It is one of 90 local authorities using the software, which failed to identify those who did not want their details passed on at the end of the annual canvass this year.
Andrew Moulton, the council’s head of governance and improvement services, said: “I’d like to apologise to affected residents. As soon as we were made aware of the matter, we took swift action to remove names and addresses from the public domain.
“We have since provided the correct information to the various agencies that sell on this type of information and we believe personal details have now been removed.
“I’d like to reassure our residents that every step necessary will be taken to ensure this doesn’t happen again.”
Residents with concerns can call 0118 974 6000. The incident has been reported to the Information Commissioner’s Office.