Childcare professionals can now access the details of every youngster in England using a controversial government database.
ContactPoint, which holds the names, ages, addresses and other information of an estimated 11 million children, is now operational.
The system cost £224m to set up but has been hit by a series of delays due to fears over security. It was launched in response to the death of Victoria Climbie, who was abused by her great aunt and her boyfriend.
Around 800 practitioners, including social workers, police and healthcare professionals, will now begin training in order to learn how to use the database.
As well as names and addresses, the system also holds details for the child's school, GP and other healthcare services, such as social workers.
More than 51,000 children deemed vulnerable have had their identities and information shielded, the government said, after fears were raised about unauthorised access to data.
Children's minister Delyth Morgan said: "Under current arrangements, if a practitioner believes that a child is at risk or may need additional support, for example if they have a disability, they may have no way of knowing whether other services might already be in contact with that child."