Parents will have a harder time finding out if any paedophiles live nearby after a High Court ruling that sex offenders' human rights to privacy should be respected.
In a judgement that appears to strike a blow against child protection measures providing greater transparency, such as Sarah's Law, the court decided offenders had the right to a say before their presence was disclosed.
Their human right to a family life should also be taken into consideration, the judges said.
In a test case brought by a repeat sex offender from the Sheffield area, two judges agreed he had the right to make "representations" to South Yorkshire police before local parents who made inquiries about him were told of his criminal past.
Sarah's Law, introduced last year, allows parents to find out from police if someone has a record for child sexual offences.
Known formally as the child sex offender disclosure scheme (CSOD), it is named after Sarah Payne, who was murdered in 2000 at the age of eight by Roy Whiting, a paedophile.
But the new ruling could force a redrafting of the law.
Sir John Thomas, one of the judges, ruled that key parts of the Home Office guidelines were "unlawful" and said a "balancing exercise" was needed between sex offenders' human rights to respect for their privacy and family lives on the one hand, and the interests of protecting children on the other.
Observing that criminal records and other information about sex offenders were often inaccurate, the judge said that in most cases it would be necessary to ask sex offenders if they wanted to make representations before their details were revealed to members of the public.
Sitting with Mr Justice Hickinbottom, he said: "Whilst each case will turn on its own facts, it is difficult to foresee cases where it would be inappropriate to seek representations, unless there was an emergency or seeking the representations might itself put the child at risk."
He added: "We conclude that the CSOD guidance sets out a policy and process that in a number of specific cases may well not comply with the applicable legal principles".
The judges stopped short of overturning the CSOD scheme altogether, saying that would be "disproportionate and risk harm to children".
But their comments could require the re-writing of the guidance to ensure child sex offenders' rights are respected.
The man who brought the case to court, referred to only as "X", is on the sex offenders' register for life.
He admitted two indecent assaults on a child at Sheffield Crown Court in 1990 and was jailed for two years. In 1996, he pleaded guilty to four more sex attacks on a child and received a four-year prison sentence.