Sex offenders could soon be asked to undergo regular lie detector tests to see if they are at risk of committing further crimes.

The head of the National Probation Service, Eithne Wallis, said it was considering the use of polygraph machines following a series of successful trials last year.

It is thought the tests, involving 30 sex offenders in three English counties, prevented at least three of them committing new attacks.

Ms Wallis told BBC Radio 4's Today programme that more research was needed, but lie detectors could be used alongside existing methods to help manage paedophiles and other sex offenders in the community.

Victims

Forensic psychiatrist Professor Don Grubin, who oversaw the tests said convicted paedophiles had revealed unauthorised contact with children when the lie detectors were used.

They are very deviant types of individual and are usually very skilled at being manipulative

Polygraph examiner

He said the men, in Surrey, Northumberland and the West Midlands, admitted visiting areas where they might meet children.

"They were beginning to go in search of victims," Professor Grubin said.

"They hadn't offended yet but had no doubt in three of these cases they would have done."

Heart rate

The sex offenders, who were all on probation, were asked a series of questions about their past offending, current behaviour and fantasies.

They were asked if they had been in contact with children or on the look-out for victims.

The polygraphs measured their physical responses, including heart rate, to see if they were lying and help build up a clearer picture of their progress on treatment programmes.

Paedophilia provokes a great deal of anger

Sandy Gray, a polygraph operator in Phoenix, Arizona, told the BBC she believed it was a safer way of keeping track of a paedophile's behaviour.

She said: "They are very deviant types of individual and are usually very skilled at being manipulative, but monitoring via polygraph is far better than accepting a self report, or them telling you what they have been doing."

And UK-based polygraph examiner Bruce Burgess said: "They have a reputation as not being seen as reliable, but I believe they are as reliable as other methods of catching criminals, certainly more reliable than handwriting or eye witnesses."

'Scrutiny'

Ms Wallis agreed polygraph testing could eventually be used by the Probation Service to monitor sex offenders, but more research was needed first.

They are so devious I wonder whether technology can keep pace with them and effectively monitor them

Roger Stoodley

"If it worked and if there was a robust, demonstrable level of reliability it may be that there would be a place for that as a piece of supporting information in our management of sex offenders," she said.

"In some other countries lie detector tests have been used, and various claims are being made about them.

"We will see whether or not these claims that are made stand up to scrutiny and we will begin to think through do they have a place in the management of sex offenders or do they not."

It is understood the Home Office is considering further trials of lie detectors on a larger scale - but plans are still in their early stages.

Practised liars

Doubts over the accuracy of polygraphs mean they cannot currently be used by police and are not accepted as evidence in the courts.

Among those voicing concern about the reliability of lie detectors is Roger Stoodley, who led the investigation into a paedophile network which included the notorious child killer Sidney Cooke.

Mr Stoodley told Today that sex offenders are practised liars and would be able to fool the most sophisticated equipment.

"I will support anything that will monitor sex offenders but I doubt that a polygraph is an efficient way of doing it," he said.

"Most sex offenders will even deny that they have done anything wrong, and indeed a lot of them blame the victim.

"They are so devious I wonder whether technology can keep pace with them and effectively monitor them."