Children may be at risk of abuse because of a 'postcode lottery' in disclosures made under the paedophile warning system known as Sarah's Law, campaigners claim.
In some parts of the country as few as one in 30 requests to police lead to the identification of child sex offenders or others who may harm children, research by the NSPCC suggests.
The Child Sex Offenders Disclosure scheme was rolled out across England and Wales in April 2011 following a campaign by Sara Payne, whose daughter Sarah was murdered by convicted paedophile Roy Whiting in 2000.
It allows parents and guardians to ask police if someone who has contact with their son or daughter has been convicted or suspected of child abuse, but disclosure is not guaranteed.
No information is given if an individual has no relevant convictions and there is no significant police intelligence.
Data obtained by the NSPCC indicates that since the scheme was introduced nationwide, one in five applications has led to information being supplied.
Between 2011 and 2014, a total of 4,332 requests were made to 33 police forces and 870 resulted in a disclosure.
The figures, obtained under Freedom of Information laws, revealed wide regional variations.
Between 2011 and 2014 South Yorkshire Police received 175 applications from people wanting information and disclosed details of known offenders on 25 occasions.
But the force has stressed that on 114 other occasions over the same period it disclosed information about officers' concerns for children without anyone making an application.
NSPCC chief executive Peter Wanless: "Families need to know if there are individuals in their area who pose a risk to children. How can you expect parents to make the right choices in order to protect their children if they don't know who is a threat?
"The wide variation in disclosure numbers doesn't breed confidence that the scheme is being understood or applied consistently and that is a concern.
"While there may be very good reasons for not disclosing information held to applicants, some forces seem to be too cautious which could put children at serious risk of harm."
However, Simon Bailey, NPCC lead on child protection, said the release of the figures is 'unhelpful and potentially misleading'.
Mr Bailey said: "It follows that where an application was made but no information on that person existed, that would not be logged as an occasion where information about someone who posed a risk to children was provided to the applicant. The reason for this being quite simply that there was no information to disclose."
Forces take into account 'community considerations' such as risks to life and vigilantism when assessing applications, he said.
"We also have to accept that on occasions the process is being abused by applicants who have ulterior motives and are simply seeking reassurance about individuals connected with their children," Mr Bailey added.
"I am assured of a consistent approach in dealing with 'Sarah's Law' applications across forces and can reassure the public of the police service's absolute commitment to the protection of children."
Detective Chief Inspector Sarah Poolman, South Yorkshire Police's lead officer for child protection, said: “Police and partner agencies across the country are absolutely committed to safeguarding children and the NSPCC provides an invaluable service to children and young people suffering from harm or abuse.
“However, the figures issued today are somewhat misleading and do not take into consideration the fact that, in South Yorkshire, the force makes a considerable effort to proactively disclose information to specified members of the public when children are deemed to be at risk, rather than waiting for a member of the public to ask us for this information.
“We have proactively disclosed information of this nature on 114 occasions to specified members of the public, demonstrating our commitment to keep the public informed and protect children from risk.
“The low application response rate referred to could simply be because we hold no information on those particular individuals, so there was nothing to disclose.
“In addition, whenever we receive an application under Sarah’s Law, a comprehensive assessment of that request must take place where we primarily look at risks to children and balance this with other considerations such as risks to life. This process is carefully managed to ensure that information is disclosed appropriately and when necessary to protect children.”