New analysis has found that 150,000 children in the South East are living in poverty but do not qualify for free school meals.
The research was conducted by Child Poverty Action Group (CPAG), which said the findings are due to "restrictive and outdated" eligibility criteria.
CPAG is calling on the Government to reform the qualifying criteria and remove means-testing from school meal provision.
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This would allow more children to access the food they need and give struggling families some financial relief.
The charity also wants an auto-enrolment system to be set up immediately to ensure no child currently entitled to free school meals misses out due to paperwork issues.
In England, children in reception, year one, and year two automatically qualify for free school meals.
However, children in year three and above from families on universal credit only qualify if their household income is below £7,400 per year, before benefits and after tax.
This threshold has been unchanged since 2018, despite rising inflation.
Kate Anstey, head of education at CPAG, said: "It’s hard to focus on your times tables when you’re hungry at school.
"But in every corner of the South East, kids are going without the food they need because the qualifying threshold for free school meals is out of date and much too restrictive.
"In line with its mission to tackle child poverty and improve children’s well-being, the Government must urgently make free school meals available to every child that needs them and work towards removing means-testing entirely from the dining hall.
"This would take some financial pressure off struggling families and give them peace of mind, while helping to ensure that every child has what they need to make the most of school.
"We should be giving children the nourishment they need to succeed, not settling for what they can get by on."
The charity says that government action on free school meals in England "lags far behind" other UK nations.
In Wales, universal provision is now fully rolled out in primary schools, and in Scotland, universal free school meals are offered to all children in the first five years of primary school.
In Northern Ireland, the eligibility threshold is double that of England, set at £15,000.
Some areas in England have also expanded provision.
Emergency funding from the Mayor of London, Sadiq Khan, means all state primary school children in London will receive free school meals until the end of the 2024/25 academic year.
The Mayor has also committed to extending the policy beyond.
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