Almost 12,000 schoolchildren are officially classed as missing from school, a TES investigation in England has found.
The numbers have triggered growing concern for the "lost pupils", with children's charities and Ofsted inspectors warning they are at "serious risk" of physical, sexual and mental harm.
The statistics, obtained from every local authority south of the border using the Freedom of Information Act, shows 11,911 pupils are missing from school. Most of them are from large urban areas.
There are a number of reasons why children may be registered as "missing from education", which describes children who have not had any kind of schooling for a month. The explanations for this can range from being forced to wait for a school place after moving house to being withheld from formal education by parents.
However, the new figures show that nearly 1,500 children have completely dropped out of the school system and cannot be traced.
Leicester, with 2,611 pupils missing from education, has the highest number. Eight other authorities; Birmingham, Kent, Bradford, Brent, Sheffield, Southampton, Doncaster and Westminster, have more than 360 missing children. Some 69 local authorities claim to have none.
Every council in England is legally required to record the number of children in the area absent from school, but there is no national system to help officials trace them or to share information with another local authority.
Martin Narey, former Barnardo's chief executive, described the figures as "deeply troubling".
"School is somewhere that every child needs to be every day," he said. "For many of our most vulnerable young people, it is the only stability they know, the only time when a little chaos is taken out of their lives, the only time when they are required to behave reasonably.
"It is the one place where poor life expectations can be reversed. So for thousands of children - inevitably those most in need of education - to be missing from school is deeply troubling.
"We need to keep children in school or, when removal is necessary, as it sometimes is, ensure alternative provision is always made available in a timely manner."
Enver Solomon, director of policy for The Children's Society, said: "It is vital children don't disappear from the school roll. There's a danger this could happen to vulnerable pupils if schools just focus on attainment and their welfare is overlooked.
"The most marginalised children have the most complex needs: they must be given additional help to remain in education."
The last official estimate from the Government - made five years ago - estimated there were 10,000 children missing from education in England.
A Department for Education spokeswoman said: "We expect local authorities to follow procedures to identify children in their area who may be out of school, ensure that these children are not missing out on their education, and allocate a place as quickly as possible."