Probation union Napo expressed disbelief on Sunday at proposals to mechanise key aspects of the service by using machines instead of officers to assess criminals.
The initiative, which is already being used in the US, will reduce face-to-face contact between offenders and probation staff, with freed prisoners and those on community orders answering questions automatically posed by a machine.
Napo warned the scheme would damage public confidence in the supervision of offenders.
An internal policy document acknowledged the trial may have "some disadvantages," including the danger that a machine will be unable to spot early warning signs of offenders posing an increased risk.
And the lack of personal contact on the so-called biometric reporting scheme may reduce the support offered to offenders.
Napo assistant general secretary Harry Fletcher said the proposal - initially dismissed by many staff as a hoax - was "extraordinary" and would damage public confidence in the probation service.
The pilot scheme, which will apply to all offenders including paedophiles, terrorists and murderers, is expected to take place in the London boroughs of Bexley and Bromley later this year and may last up to six months, staff have been told.
Higher-risk offenders could be asked to use the machines, which are equipped with fingerprint readers, in addition to face-to-face interviews with probation officers.
According to the document, probation officers "will use their professional judgement to determine to what extent it forms part of an offender's reporting requirements."
It added that every offender will continue to have "an appropriate level of face-to-face supervision."
The machines will ask offenders a series of questions, including whether they have changed address or employment, if they have been arrested, or if they wish to speak to someone.
But Mr Fletcher said there was the risk that some offenders may be able to manipulate the system by lying and falsely suggesting they were complying with orders.
"The introduction of machines rather than people into the supervision of community orders made by the courts or of people on licence is extraordinary and defies belief," he said.
London Probation Trust claimed that the initiative was a research project which would "explore the potential use of biometric technology within probation."
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