A list of suspected criminals was mistakenly emailed to 150 farmers by Northumbria police, attached to a Farmwatch bulletin.
The regular newsletter, sent on Friday (14 November), contained details of 51 local people suspected of being criminals as well as what action police planned to take against them.
One farmer said he had been stunned to see the names of three people he knew.
An inquiry has begun into how the lapse in data security happened, and the Information Commission has been informed.
Northumberland area commander Chief Superintendent Kevin Lambert told the Daily Telegraph: “As soon as the error was highlighted to Northumbria Police management, all necessary steps were taken to contact those individuals who had received the email, asking them to delete it immediately and not to open or read the attachment.
“We sincerely apologise for this error.”
Mr Lambert said he didn’t believe individual or public safety had been compromised at this stage.