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.
However, the National Police Chief's Council (NPCC) described the figures as "potentially misleading".
The Child Sex Offenders Disclosure (CSOD) 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.
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.
The NSPCC said 3% of the requests made to Lancashire police led to information about people who pose a risk to children being given to applicants.
Sussex Police, which investigated Sarah Payne's murder, made disclosures in one in 14 instances (14 out of 193 applications), according to the charity.
By contrast, Cleveland police made disclosures in response to 131 out of a total of 147 requests - a rate of 89%.
NSPCC chief executive Peter Wanless said the body was "disturbed and surprised" by the "wide discrepancy" of figures.
He said: "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".
Forces were asked how many applications they received under the CSOD and the number of occasions when information about someone who posed a risk to children was provided.
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."
Once a request is made under the CSOD police assess if it raises concerns - including the discovery that the subject has child sex convictions or a criminal record for other relevant offences such as violence, drugs or domestic abuse. Any subsequent disclosure is strictly confidential.
No information is given if an individual has no relevant convictions and there is no significant police intelligence.
Five forces received a total of more than 900 requests but did not provide information about disclosure numbers. Four did not supply any data and one had recorded no applications.
Detective Chief Inspector Pierre Serra, of Sussex Police, said: "Out of the 193 applications, 14 were given disclosures. Of the remainder there may have been information already shared with partner agencies, no information about that person to disclosure, information may already be in the public domain and at times we have made disclosures to family members who have direct responsibility for a child.
"Each case is assessed after careful consideration, thorough checks and risk assessments."
A Home Office spokeswoman said: "The UK has some of the toughest powers in the world to deal with sex offenders. This includes the Child Sex Offender Disclosure Scheme."