Doctors, social workers and police will now have access to the details of every child in England following the launch of a controversial database.
It has been designed to give organisations and health professionals quicker access to information to help children who are at risk.
ContactPoint was set up following a recommendation by the Laming Inquiry into the death of eight-year-old Victoria Climbie in 2000.
The £224m directory will contain the name, address, date of birth, GP and school of all under-18s, as well as the contact details of any professional who is working with the child.
The database has been established to help professionals share information on young people to stop children falling into gaps between different services. ContactPoint will not hold case information and users will not be able to download its contents.
However, the project has been delayed because of worries over data security and technical issues with civil liberties groups, children's campaigners and the Office of the Information Commissioner expressing concerns.
Children's secretary Ed Balls said it would help those who work with children to "prevent problems escalating". He said recent cases had shown that a lack of "proper and timely information sharing" could have tragic consequences.