Parents in four areas of England will be able to check under a pilot scheme launched yesterday, whether new partners or other people with significant access to their children are sex offenders.

The initiative allows police and probation services to disclose some information to families about people who have unsupervised contact with their children.

But the scheme, which will operate in Cambridgeshire, Hampshire, Cleveland and Warwickshire, does not go as far as the so-called "Megan's Law" in the United States which allows local identification of offenders.

"You have to be a parent, carer or a guardian and you would go to the police or the authorities and say you have concern about somebody who had unsupervised direct access to your children," Home Office minister Vernon Coaker told BBC radio.

"It may well be a babysitter, it may be a new boyfriend, it may be somebody who lives next door but it has to be somebody who has that unsupervised access."

When the proposals were announced last year, Home Secretary Jacqui Smith said nine out of 10 cases of child sex abuse were committed by someone known to the child.

The move has been welcomed by Sara Payne, whose 8-year-old daughter Sarah was abducted and murdered by a convicted paedophile in 2000 and who has since campaigned for the whereabouts of convicted sex offenders to be made public.

"This is a giant step towards truth and honesty when dealing with sex offenders and all we need now is for local communities up and down the UK to help make this work," Payne said.

However, probation officers and charities have expressed concern that the measures could lead to vigilante attacks, forcing paedophiles underground.

They also warn that predatory paedophiles take time to groom victim's families, picking on the most vulnerable who are unlikely to risk any new relationship by making such check-ups.

"They are not going to check on potential partners if it means the relationship might be disrupted," said Harry Fletcher, of Napo, the trade union and professional association for family court and probation staff.

Coaker said there would be safeguards in the system to ensure information was kept confidential.