Last month thieves stole IT equipment containing both databases of offenders and personal information about probation service staff, including details of their bank accounts.
Nottinghamshire chief probation officer David Hancock informed staff of the theft, but not offenders. In a letter to Mansfield solicitor Mark Marriott, Hancock, said, "While we cannot be absolutely sure, our current assessment is that the theft was targeted at the equipment rather than the data."
"I have reviewed the question of whether or not offenders, and other service users, should be informed of the theft. "I have judged that it is inappropriate to do so in the absence of any evidence about the data being targeted or leaking."
His reply failed to satisfy local lawyers, the data protection commissioner or the probation officer's union NAPO.
Marriott said, "Stolen data has a habit of leaking out and the Probation Service has a responsibility to victims and offenders to inform them."
Amanda Chandler, compliance officer at the Office of the Data Protection Commissioner, said the Probation Service was not exempt from data protection laws. "We would expect probation officers working with individuals to let them know that the data is missing."
Harry Fletcher of NAPO, described the theft as "an appaling lapse. The Nottinghamshire database will contain some of the most sensitive cases in the country," he added.
The Nottinghamshire Probation Service refused to comment.