Authorities in the mid-1990s lost track of a number of criminals freed on parole from life sentences, a Home Office report has revealed.
Offenders were lost after release on parole from life sentences
Researchers could not trace 20 of 138 offenders, including murderers, rapists and arsonists, freed on licence.
Conservative home affairs spokesman Nick Herbert called for a review of the probation system.
A Home Office spokesman said the system now used a database introduced since the period of the study.
Mr Herbert told BBC Radio 4's The World Tonight that the National Probation Service was in danger of losing public confidence.
He said: "Some of the incidents of offending that has happened on early release and the figures for the extent of breaches of some of the non-custodial regimes, for instance people on tags, have undermined that confidence and I think, therefore, that the whole system needs review."
Home Secretary Charles Clarke earlier this week ordered an inquiry into the killing by two convicted criminals of London banker John Monckton in his own home in November 2004.
Damien Hanson, 24, convicted of murder, was being managed as low-risk by the National Probation Service after release from a 12-year term for attempted murder despite having been assessed as 90% likely to commit more violence.
Elliot White, 24, convicted of manslaughter, was also being supervised by probation officers while on bail on drugs charges.