People accused of sex offences in Hertfordshire could face a lie detector test under a new pilot scheme being run by the county's police.

The force is trialling the use of polygraph machines when questioning suspected sex “low level” sex offenders.

The machines measure vital signs such heart rate, brain activity, sweating and blood pressure, during questioning, which is interpreted for signs about how truthful the subject is being.

Hertfordshire Constabulary said the technique has thus far only been used on 25 suspects who had volunteered to undergo it.

The polygraphs took place before any charges were brought and conducted by specialist officers from the constabulary’s Paedophile Unit working with an expert.

The force said the method was used to speed up its “risk assessment process” and that evidence elicited during the examinations is not admissible at court. Detective Chief Inspector Glen Channer from Hertfordshire Constabulary said: “The Polygraph Testing provides us with an additional tool and has cut down investigative time significantly leading to a more efficient process, often helping to identify additional offences. The integrity of the process is paramount and we are working with a highly credible expert in the field on this trial, Professor Grubin from Newcastle University.”

However the Association of Chief Police officers urged caution with forces adopting the device, saying was not the single solution to solving crimes.

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."