One in 10 calls to police under a "Sarah's Law" paedophile scheme uncovered the hidden criminal past of someone with access to a child, it has been revealed.
The largest number of queries came from fathers worried about their ex-partner's new boyfriend, police said.
Ministers confirmed the new disclosure rules will apply to all forces in England and Wales from March next year.
The year-long trial began in September 2008 involving the Cambridgeshire, Cleveland, Hampshire and Warwickshire police forces.
It is a watered-down version of laws in the US where details of where convicted paedophiles live are actively publicised.
As the scheme is extended, every one of tens of thousands of registered sex offenders will be contacted to reassure them it will not lead to public vilification.
Sara Payne, whose daughter Sarah was killed by convicted paedophile Roy Whiting in July 2000, led a high-profile campaign calling for a British equivalent.
Under the Home Office scheme parents can ask the police about anyone with access to their children and officers will reveal details confidentially if they think it is in the child's interests.
Police may also warn parents if concerns are raised by grandparents or neighbours.
Ms Payne, who is now the Government's official Victims' Champion, and is recovering from brain surgery, welcomed the announcement.
In a statement, she said: "I am delighted that the years of campaigning and hard work by so many friends and colleagues have provided those who care for children with the right to check that adults who have access to them do not pose a danger.
"The evaluation has shown the huge benefits of increased but controlled access to information."
It was predicted many of the requests for information would come from women concerned about the history of their new partners.
But police found nearly one in five applications was from parents asking about a new lover in their former partner's life.
Neighbours, other family members and friends were the other main subjects of inquiries.
The forces involved in the pilot were not deluged with requests as was feared.
Worries that paedophiles would disappear underground en masse and that the revelations would result in vigilante justice were also not realised, police said.
A total of 315 applications for information were made to the four forces.
From those, details of 21 paedophiles were revealed. Details of 11 individuals with different criminal pasts such as violence were also divulged.
An academic study into the results of the pilots said they strengthened existing controls on sex offenders.
In one case a neighbour giving sweets to youngsters was found to be a paedophile. He was arrested for breaching a court order which barred him from contact with children.
Chief Constable Paul West from the Association of Chief Police Officers said contacting every individual sex offender to warn them about the scheme helped reassure them it would not lead to them being publicly outed.
"There has been no evidence to suggest that any registered sex offenders in the areas have gone missing and been driven underground," he said.
"The preparation for rolling out the process involved meeting every registered sex offender in the area to explain this wasn't going to be a public outing and it wasn't going to be front page news in the local paper. I think this gave a lot of reassurance."
There are currently 32,336 registered sex offenders in England and Wales.
Home Secretary Alan Johnson said: "I am determined to do all I can to protect children and families from sex offenders.
"The UK already has one of the most robust systems in the world for the management of sex offenders. The new scheme will build on this, ensuring more children are kept safe.
"We've already seen that children are better protected and sex offenders more effectively managed because of this scheme, which is why it is rolling out nationwide."
The scheme will begin in 18 new force areas from August, the Home Office said. They are: West Midlands, Essex, Bedfordshire, Dorset, Durham, Thames Valley, Suffolk, Norfolk, Lincolnshire, West Mercia, Leicestershire, Northamptonshire, Surrey, Sussex, Wiltshire, Staffordshire, Cheshire and North Yorkshire.
It will then extend across England and Wales from March 2011.
Shaun Kelly, head of safeguarding at the charity Action for Children, said it was important the project was properly resourced to make sure it was appropriately monitored.
He said: "Any measures that help to protect children and young people from sex offenders are welcome, and this is certainly a step in the right direction.
"Going forward however, the success of this project lies within the commitment from Government to properly resource this programme to ensure it is successfully rolled out and, crucially, appropriately monitored."