Tens of thousands of England's poorest families are missing out on additional support for their children, a report into school subsidies has found.
Academics say as many as 4% of school pupils - around 200,000 across the country - are "super-deprived", and are failing to receive the free school meals (FSM) to which they are entitled.
Their schools are also losing out on crucial Pupil Premium payments, distributed by the Government to help support the most disadvantaged children, according to the University of Birmingham's School of Education.
Free school meal data is routinely used as a measure of low parental income
Professor Stephen Gorard, who led the research, said some of society's poorest children were being let down by an "unintended, unwanted" situation in which some schools were failing to recognise the extent of some pupils' poverty.
He said: "There was a time when village schools knew every pupil, and they knew those who were most in need of support.
"But, for many reasons now, we have a situation where pupils are moving to new schools without the paperwork to state their support needs, whether or not they need free school meals, whether their school will qualify for the pupil premium.
"These pupils are potentially among the most impoverished in England, so it is in everyone's interest - the pupils, their parents, the schools and the Government - to address this. It is nobody's fault, really, that this has happened. But any solution would be a win-win situation for everyone."
It is also crucial to the Pupil Premium, additional school funding for those in most need of extra support.
The report states: "Missing FSM pupils are a third category of low achieving students, often not taking traditional qualifications at Key Stage 4, who are 'mobile' between schools and about whom their schools know less.
"They are frequently disadvantaged in other ways as well and have some appearance of a super-deprived and very low achieving group."
Prof Gorard added: "There is a danger that three-to-four percent of secondary school pupils in England are not being correctly identified as being eligible for free school meals and their schools are missing out on the Pupil Premium as a result.
"Schools are in danger of being doubly disadvantaged. Not only may they miss out on their correct Pupil Premium, the context of their attainment results, as judged by Ofsted and others, may not reflect the true picture."
The Department for Education agreed with Prof Gorard over the need to ensure children eligible for free meals were given the extra support.
A spokesman said: "We are writing to local authorities with low take-up to remind them of the responsibility they share with the schools in their area to make sure that every eligible child is signed up.
"We want every child eligible for free school meals to sign up for them - so that pupils benefit from a nutritious meal everyday and schools receive all the Pupil Premium funding they need to support them.
"We are working with the Children's Food Trust to increase the take-up of free school meals and we have streamlined the application process so that parents can check their own eligibility and apply online."
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