Over 600 staff at HM Revenues & Customs (HMRC) have accessed sensitive data with no authorisation, over the past three years.
The employees have since been disciplined or fired for their actions, but worries have arisen about the safety of the public's information.
Jane Kennedy, financial secretary to Treasury, said that people had been given a warning or fired after "inappropriate access to personal or sensitive data".
"Breaches of this policy are taken seriously and any breach will result in the commencement of disciplinary proceedings," she said.
All the incidents occurred between April 2005 and December last year.
Ms Kennedy said the organisation now has more stringent controls in place to ensure the safety of data.
Following the loss of 25 million child benefit records last year, the authority has stopped bulk transfers of data unless "adequate security protection" is put in place.
Recently a member of the House of Lords, Merlin, the Earl of Erroll, said HMRC's loss occurred because of targets, meaning the data had to be moved quickly.
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