Image caption Lie detector tests are considered too unreliable for use in criminal trials in the UK

Police have begun using lie detector tests on suspected sex offenders in a trial which could be widened.

Hertfordshire Police said it had been using polygraphs, which monitor heart rate, brain activity, sweating and blood pressure, during questioning.

The trial involved tests on 25 "low-level" suspects ahead of any charges being brought, the force said.

The Association of Chief Police Officers (Acpo) said the tests were not a "single solution to solving crimes".

Hertfordshire Police said polygraph testing had been used to "speed up the risk assessment process".

"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 suspected offenders who have volunteered to co-operate with police," the force said in a statement.

"Evidence elicited during the examinations is not admissible at court."

Det Ch Insp Glen Channer, head of the force's child protection unit, added: "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."

A further 12-month trial is expected to start in April, the force says.

'Home in'

Acpo, which represents chief police officers in England, Wales and Northern Ireland, said its Homicide Working Group advised police on the use of polygraph techniques.

"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," a spokeswoman said.

If they polygraph six people and they get five truthful results and one deceptive, they can home in on that person Bruce Burgess, Formerly of the British Polygraph Association

"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."

The results of lie detector tests are considered too unreliable for use in criminal trials.

Bruce Burgess, a former chairman of the British Polygraph Association, said lie detectors were considered to be a useful "investigative tool" in the US, even though they produced evidence that was "very difficult to get into court" and were unlikely ever to be used as "a guilty or innocent tool".

He said offering polygraph tests to a number of suspects could provide a degree of insight if, for example, somebody refused to take the test.

"If they polygraph six people and they get five truthful results and one deceptive, they can home in on that person and cut down on a lot of police work. That's the way it's used in America," he added.

Although the Hertfordshire pilot looked at the use of lie detectors to aid decisions over whether or not to charge suspects, a three-year pilot study in the East and West Midlands could lead to the compulsory testing of convicted sex offenders.

The Ministry of Justice has been overseeing the project, aimed at testing sex offenders as part of their probation conditions when they are freed from prison.