Childcare experts have raised concerns over plans to stop police being called out to around a third of missing persons cases.
The NSPCC warned that the changes could put children at risk of being sexually exploited, while the Children's Society claimed that pilots carried out were too short to prove the plans were safe.
Under the changes, call handlers will class cases as either "absent", when a person fails to arrive somewhere they are expected, or "missing", where there is a specific reason for concern.
Police deal with around 327,000 reports of missing people each year, the equivalent of around 900 a day, two-thirds of which involve children.
There is often a link between a child frequently going missing and falling prey to sexual abuse.
David Tucker, head of policy at the NSPCC, said the charity fears the new definitions could put children at risk, saying: "We are very concerned that the new definition of 'missing persons' will put vulnerable children at risk of being groomed and sexually exploited."
"The length of time a child goes missing is irrelevant because they can fall into the clutches of abusers very quickly.
"Children go missing for a variety of reasons - they may be bullied, abused or are generally unhappy. But whatever the reason, this problem must be taken seriously. We expect all professionals including the police to invest the right amount of time and take the necessary action to protect all children as soon as they go missing."
Chief Constable Pat Geenty, the lead for missing people for the Association of Chief Police Officers (Acpo), insisted the move will free up officers' time and improve the way forces deal with children who go missing from care. He said: "There is an element about reducing bureaucracy, but I am convinced that the change will enable us to focus resources to protect those children that we need to protect."
Pilots of the new system have been carried out in Greater Manchester, West Midlands and Staffordshire, and Sussex Police have been using the definitions for three years.