The probation service is considering plans to force sex offenders to undergo lie detector tests.

The head of the National Probation Service, Eithne Wallis, confirmed on Thursday that officials are considering widening a pilot programme where offenders considered likely to re-offend are subjected to polygraph tests.

Wallis told BBC Radio's Today programme that the idea was being considered though further research would be needed after widening the pilot project.

"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."

The current pilot schemes in Surrey, Northumberland and the West Midlands are being conducted by forensic psychiatrist professor Don Grubin who claimed it had prevented re-offending.

Several paedophiles had admitted to visiting areas where they might meet children.

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

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

The idea is subject to serious debate among experts.

Polygraphs are routinely used in America, including part of sex offender programmes. They are not judged to be reliable enough to be used in British courts or by the police.

Some experts who work with sex offenders claim it would be easy for paedophiles, who are often accomplished liars, to beat the tests.

Roger Stoodley, who headed the investigation into killer Sidney Cooke doubted whether testing would work."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."

But Grubin, who is head of the sexual behaviour unit at Newcastle University, said the machines were not easy to fool.

"Of course anyone can beat the polygraph on an occasion but around 90 per cent of the time people cannot and that is the sort of level that we need."

"You have to remember the polygraph is just one in a series of tools being used to monitor sex offenders and as an addition it certainly seems to be a very useful one."

Civil liberty groups questioned the reliability of the tests.

Roger Bingham of Liberty said: "It seems reasonable to use polygraphs if they are proved reliable. However, they should not become a substitute for other safeguards used to monitor such offenders. There is always a danger if you rely too much on one thing."