Suspected sex offenders have been targeted with lie detector tests in a police trial to gauge the risk they pose to the public.

Hertfordshire police has been using polygraphs, which monitor heart rate, brain activity, sweating and blood pressure, during questioning to help decide whether to charge suspects.

The force confirmed it had completed a successful pilot scheme in November in which 25 "low level" sex offenders were tested. Many were found to pose a higher risk to children than previously estimated. A further 12-month trial is expected to start in April.

Of the 10 offenders tested in April, six revealed more serious offending. Testing ended so further inquiries could take place, according to the Times. Of the other four, two revealed offences, thus passing the test, and received cautions and attended a sexual offender treatment programme.

The last two made disclosures and passed the test but refused to admit the offences when interviewed later. Their cases went to court. All four were placed on the sex offenders register.

Of the 15 offenders tested in November, eight failed while six passed. One was caught trying to beat the polygraph, breathing erratically and talking slowly.

Detective Chief Inspector Glen Channer, head of Hertfordshire's child protection unit, said the polygraph was an "added weapon in our armoury of investigative techniques". He said the tests were carried out by accredited practitioners in a scientific environment and were not relied upon on their own.

Hertfordshire police said : "The testing is undertaken ahead of any charges being brought and involves specialist officers from the constabulary's paedophile unit working with an expert who conducts the test on first-time offenders who have volunteered to co-operate with police. Evidence elicited during the examinations is not admissible at court."

The Association of Chief Police Officers (Acpo) cautioned that the tests were at a very early stage. A spokesman said: "Polygraph techniques are complex and are by no means a single solution to solving crimes, potentially offering in certain circumstances an additional tool to structured interrogation.

"These initial trials are in their very early stages and we will follow their progress, working with chief officers across the country to provide further guidance if necessary.

"Whether these techniques are adopted elsewhere in the country is a matter for individual chief constables."