Plans for criminals to report to a machine instead of a probation officer in a bid to cut costs defy belief and risk increasing offending, a union warned today.

The initiative, which is already being used in the United States, 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.

The probation union Napo warned the scheme would damage public confidence in the supervision of offenders, but probation bosses said it could reduce bureaucracy and help staff make the best use of their time.

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.

The lack of personal contact on the so-called biometric reporting scheme may also reduce the support offered to offenders, staff have been told.

The document added: "Removal of contact may remove the potential for an early warning of escalation of risk."

Harry Fletcher, Napo's assistant general secretary, 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 killers, is expected to be trialled 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 judgment 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 scheme will be designed to test whether the move would reduce the time spent by offenders waiting in probation offices and cut the need for staff cover during peak holiday periods and sickness.

It will also test whether using the machines reduces the risk of offenders arranging for someone else to take their place and report to probation for them.

But Napo said it was not aware of any widespread use of imposters.

The union added the machines cost £130,000 a year for each London borough, taking the annual cost across the capital to £4.16 million.

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.

"When the idea of machines rather than face-to-face contact was first mooted, staff thought it was a hoax," he said.

"Sadly it is now grim reality.

"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 added that the scheme "will lead to high breach rates and a lack of confidence in the supervisory process".

"The vast majority of offenders have serious literacy problems, many are dyslexic, most have two or more mental illness and desperately need face-to-face contact and supervision," he said.

He added the machines could also breach up to 18 Council of Europe rules on community sanctions but, according to the policy document, probation bosses have been given legal advice that offenders can be required to take part in the pilot.

London Probation Trust said the initiative was a research project which would "explore the potential use of biometric technology within probation".

The scheme will not replace the trust's statutory responsibility to provide face-to-face meetings with offenders, officials said.

Heather Munro, the trust's chief executive, added: "London Probation Trust intends to research biometric reporting of offenders to support our key aims of protecting the public and reducing reoffending.

"We believe reducing the bureaucracy probation officers have to deal with, in order to increase the time spent in face-to-face meetings, is an important step.

"We are looking at various ways of doing this, from increasing the number of probation officers to investigating the use of technology to improve our ability to monitor offenders - we want to use the time of professional practitioners where it is most needed."