Sarah's Law gives mothers right to make 'sex checks' on partners

Moves to give single mothers the power to check whether new partners are sex offenders could encourage malicious campaigns against innocent men and drive paedophiles underground, the Home Office has been warned.

Thousands of parents and guardians will from today gain the formal right to ask police to look into the background of people who have unsupervised access to their children.

The scheme is being launched in four areas initially, with a view to extending it nationwide. Ministers said the initiative would help to protect children, but critics warned they could prompt vigilante attacks.

Mothers concerned about a new partner will be able to register their concerns with police who will run an initial check within 24 hours followed by a more detailed investigation into a man's background taking up to 10 days. If he is found to be a convicted sex offender, the mother will be warned and given advice on protecting her children.

Police would need to be convinced the investigation was not prompted by malice. And mothers are being warned they cannot pass on information about a man without permission by police. Those who refuse to keep it secret could be prosecuted. The moves are being launched on a trial basis over the next year in Warwickshire, Southampton, Stockton-on-Tees and Peterborough.

The initiative is a response to the "Sarah's Law" campaign that followed the murder of eight-year-old Sarah Payne by a convicted paedophile, Roy Whiting.

It falls far short of the widespread publication of sex offenders' names and addresses demanded by its supporters, but still provoked concerns among children's and penal reform groups. Barnardo's warned that existing laws were inadequate to prevent information about sex offenders being spread within communities – and to stop innocent people falling foul of malicious complaints. It also said that the moves would force paedophiles underground, making it harder to track them.

James Welch, the legal director of Liberty, added: "The threat of prosecuting the concerned mum who tells her neighbour is unenforceable nonsense."

Jacqui Smith, the Home Secretary, said the pilots were a "huge step forward". She said: "I want to see every child living their lives free from fear."

Sara Payne, Sarah's mother, said: "This is a giant step towards truth and honesty when dealing with sex offenders and all we need now is for local communities ... to help make this work."

In America, under "Megan's Law" – following the murder of Megan Kanka by a convicted sex offender – details of registered sex offenders are made public.