The government has admitted it cost £2.25m to send letters of apology to people affected by the loss of 25 million child-benefit records by HM Revenue & Customs.
The seven-figure bill provoked outrage from the TaxPayer's Alliance, which attacked HM Revenue & Customs (HMRC) for losing the two CDs after it posted them to the National Audit Office last year.
The financial secretary to the Treasury, Jane Kennedy, revealed the cost in written answers to the House of Commons.
Kennedy stated: "The cost of sending letters to the families affected by the recent loss of data by HMRC is estimated at £2.25m, including postage costs."
Both Conservative and Liberal Democrats MPs attacked the huge spend and said, if any of the letters had gone missing, it could have triggered a second wave of data loss.
The Treasury and HMRC defended the decision to send out the letters, saying it was right to approach those affected individually.
The revelation follows the Metropolitan Police's announcement that it would seek the full costs from HMRC for the force's hunt for the missing discs.
The bill for the search, which is now winding down, reportedly makes it the UK's most expensive lost-property inquiry.