Police are to stop attending every report of a missing person to focus on cases where people are most at risk.
There are about 900 reports a day of those whose whereabouts are unknown and police have to investigate each one, but from April this will change.
Senior officers say the current policy, which applies to England, Wales and Northern Ireland, drains resources.
The NSPCC and the Children's Society have warned the changes could put children at risk.
Police deal with about 327,000 reports of missing people every year, with two-thirds of them involving children.
Chief Constable Pat Geenty, of the Association of Chief Police Officers (Acpo), said the current regime was "a huge demand on police resources".
Under the new approach, police call handlers will divide reports into two categories.
People who are simply not where they are expected to be will be termed "absent" and the cases will be monitored.
Where there is a specific reason for concern, they will be classed as "missing" - prompting an investigation.
Pilots of the approach by Greater Manchester, West Midlands and Staffordshire police forces showed officers focused more on higher-risk incidents and saved thousands of officer hours over a three-month period. Sussex Police have been using the definitions for three years.
About a third of missing person cases were likely to be classed "absent", figures from the pilots suggested.
But the NSPCC said it was concerned the new definition would put vulnerable children at risk of being "groomed and sexually exploited".
David Tucker, head of policy, said: "The length of time a child goes missing is irrelevant because they can fall into the clutches of abusers very quickly.
"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."
Acpo said each force would have a missing persons co-ordinator whose responsibilities will include finding out if children are going missing regularly. They will work closely with care homes and local authorities to produce care plans to prevent the children from going missing.
Mr Geenty said police were, at times, used as a "collection service" for children who went missing from care homes and urged staff to "act as responsible parents".
"Our response officers do a great job, they find the young people, take them back, they go missing again. What we want to do is break that cycle. This will improve safeguarding for young people and use our resources more effectively," he said.
"This approach actually gives more care and attention for these vulnerable young people."
Ellen Broome, director of policy at the Children's Society, said pilot schemes had prioritised assessing efficiency savings rather than the safeguarding of children.
She said: "Safeguarding vulnerable children is a long-term issue, and these pilots alone are too limited to draw any definite conclusion.
"It is absolutely essential that, when these new definitions are rolled out across the country, police monitor how safeguarding is affected in each area over time and that appropriate measures are in place to protect children."
The changes are being made following cases such as the Rochdale child sex ring, in which nine men were jailed for grooming and abusing vulnerable teenage girls.
A report by the Rochdale borough safeguarding children board said girls as young as 10 were being targeted for sexual abuse having been written off by those in authority who said they believed the children were "making their own choices" and "engaging in consensual sexual activity".