The private financial details of up to 50,000 people who claim tax credits have been mistakenly sent out in the post by HM Revenue & Customs (HMRC).
Claimants were sent their annual tax credit award notice, along with personal details of other claimants.
One woman from Hyde in Greater Manchester has told the BBC her letter included her neighbour's earnings.
She also got the bank sort code and the last four digits of the bank account number of another claimant.
The HMRC said it would be apologising to all the people affected.
"Unfortunately an error has occurred in one of the tax credits print runs, causing some customer information to be wrongly formatted," said a spokeswoman.
"Investigations are under way to identify the cause of the problem and we will be contacting affected customers in writing this week, apologising and providing a corrected award notice.
"An initial analysis shows that ID theft could not result from this printing error."
Karen Lippiatt, a mother-of-four from Hyde, said: "I was disgusted that this could happen and I feel very worried about the fact that somebody has possibly got my bank details, national insurance number, income details and the names of my children.
"This is information that should not be shared and whatever the revenue may say this is a security issue and I do feel vulnerable."