The US-style polygraph machines were tested by the West Midlands Probation Service and its results have been reported to other probation services. Some are said to be interested in conducting further tests of their own.

Probation Office David Middleton explains why detector was used on sex offenders

In the week long trial, child abusers and rapists were attached to the machine and monitored while being questioned.

Each of the offenders were required to sit with tubing wrapped around their stomachs and waists to monitor breathing, a cuff was tied to their upper arms to measure variations in heart rate and sensors were fixed to their fingers to gauge pespiration.

They were then questioned for two hours in an attempt to discover whether they were co-operating honestly with their therapists.

The questioning was led by polygraph consultant and former US police officer, Dan Sosnowski.

'Control technique'

Mr Sosnowski uses what is known as the "control question" technique in which general questions on honesty and criminality are mixed with questions on the particular type of offence involved.

The idea is that people who are guilty of the specific offences react more vigorously to to the relevant questions.

A spokesman for the West Midlands Probation Service said a number of techniques were used with sex offenders and the lie detectors were being looked at as another option.

He accepted there might be concern some offenders would be able to trick the machine but thought the evidence presented suggested it was easier to trick a therapist.

Lie detectors have been widely used in the US since the 1950s but they remain controversial and their results are not always accepted by courts.