People found with child pornography in Hertfordshire face lie detector tests to identify the risk of them becoming sex attackers.

Hertfordshire Constabulary officers are now being trained as polygraph examiners to assess suspects before they are released on bail.

The initiative is based on research that showed making suspects take lie detector tests led to police making more arrests and uncovering more evidence to build cases on.

The force said the tests had also prompted them upgrade their risk assessments on some suspects.

Research by Professor Don Grubin, Andrew Joyce from Hertfordshire Constabulary and Eric J Holden, from Behavioral Measures & Forensic Services Southwest, Inc, trialled polygraph tests on 31 men arrested for downloading indecent images of children initially classified as "low risk" by the police.

But after undergoing a polygraph test, the risk rating of 11 men was increased as the tests showed that five men had previously had sexual contact with children.

Subsequent investigations into these five men led to two being charged with serious sex offences and one being identified as a wanted suspect in another country.

Only eight of the 31 men being tested were kept as "low risk."

Detective Chief Inspector Stuart Orton, who is leading the polygraph testing in Hertfordshire, said: "In many instances individuals have been arrested and are on bail waiting for the technical analysis work to be undertaken on their computer equipment.

"By agreeing to an early polygraph test we can work with the offenders and understand the stress they are under and manage them more effectively."

The training, which is being led by Professor Don Grubin, chair in forensic psychiatry at Newcastle University and an honorary consultant psychiatrist in Northumberland Tyne and Wear NHS Foundation Trust, is expected to last 11 weeks.

Professor Grubin said: "There is a good deal of evidence to show that polygraph testing has the potential to substantially increase information gain for little additional cost, and in fact can save money by enabling police resources to be better targeted and speeding up the investigative process."

Hertfordshire Police and Crime Commissioner David Lloyd said: "Managing the risk that sex offenders pose is challenging and the police service must be creative in its approach.

"Hertfordshire Constabulary has been at the forefront of this innovative project which provides specialist officers with further ways to manage risk and I’m confident in the months ahead polygraph testing will prove to be highly successful."