SEX offenders freed from prison could face compulsory lie detector tests after pioneering research in the North-East showed its value.

Home Secretary David Blunkett yesterday announced plans to use electronic polygraphs to prevent offenders striking again and to ensure they keep to release conditions.

Mr Blunkett also said satellite tracking could be used to watch paedophiles and other offenders to let police know if they had been near the scene of a crime. Sex offenders in the Northumbria Police force area have been taking part in the two-year study with the probation service and Professor Don Grubin, of Newcastle University.

About 40 of them have taken the lie detector tests as part of their treatment programmes, and before the trials end next May, it is hoped 300 across the country will have had them.

Prof Grubin, a psychiatrist and expert on sex crimes, said last night: "It is voluntary so we can't force people to do it which is one of the main issues.

"Certainly based on my experience, I would support mandatory testing.

"There are two sorts of offender.

"One group who don't want to re-offend and will engage in treatment and supervision, and they need encouragement to go along with that.

"There is another group whose main aim is to avoid being caught and continue to offend, and that's the group that doesn't come along.

"That's why the tests should be mandatory."

During the study in Northumbria - which is being duplicated in another nine areas - lie detector tests have led to one man admitting he had re-offended.

"We have had a lot of interesting disclosures," said Prof Grubin.

"The probation officers here find it very, very useful.

"The aim is not to catch people re-offending, it is to stop them re-offending."

Critics of lie detector tests say the results are unreliable, but Prof Grubin insists nine out of ten are accurate and should be used only in conjunction with other methods.

He said satellite tracking would be "another useful tool" in a comprehensive treatment programme.

Prof Grubin was asked to carry out the extensive tests after a smaller study in 2001, involving 30 offenders, interested the Home Office.