The Inland Revenue has disciplined 205 people for computer misuse, and in 1994 and 1996 dismissed offenders who were later sentenced to a year behind bars.
The figures are revealed in a House of Commons Public Accounts Committee report on "Inland Revenue: Tax Credits and Tax Debt Management''.
The all-party group said that the department would spend £31m on a new management information system by 2007 to improve performance.
In its report, the PAC said, "The department has disciplined 205 people in 2002 for all types of computer misuse, not just unauthorised browsing of tax record.
"The chairman of the board recently issued a strongly worded note to all departmental staff making it clear that computer misuse was a serious offence, ultimately punishable by dismissal.
"There had been two instances, dating back to 1994 and 1996, where departmental staff had sold information. In each case the offender was dismissed and imprisoned for a year. The department's information technology could trace who accessed which records and the system itself checked where people accessed tax records outside their immediate area of responsibility.''
The PAC welcomed the investment in a new management information system but said the Revenue must ensure it developed a "good-quality system'' for the £31m.
"To maintain the confidence of taxpayers, the treatment of unauthorised staff access to taxpayer information as a disciplinary offence should be upheld, taking account of changing risks and challenges posed by changes in information technology," it said.