Criminals will report to a machine, instead of a probation officer, in a new pilot scheme in the London Boroughs of Bexley and Bromley.

The initiative, which has already been trialled in the United States, is being introduced in a bid to cut costs. But union bosses have warned that it could lead to an increase in offending.

The machines will reduce face-to-face contact between offenders and probation staff, with criminals answering a series of questions posed automatically on a screen. The devices are also equipped with fingerprint readers.

The probation union Napo has raised concerns that the machines will fail to spot early warning signs that an offender may pose an increased risk.

Assistant General Secretary Harry Fletcher said: "When the idea of machines rather than face-to-face contact was first mooted, staff thought it was a hoax.

"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."

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

Probation staff will be expected to use their professional judgement to determine whether offenders should use the machines, and all criminals will continue to have some face-to-face supervision.

"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."