All children in families receiving Universal Credit must be given free school meals, unions and school leaders have demanded.
Boris Johnson is facing urgent calls to expand free lunch provision as cost of living pressures mean more Brits struggle with feeding their families.
One in five pupils in England - or 1.74 million children - already qualify for free school meals - with more than 420,000 youngsters falling into this bracket since the start of the pandemic.
Pupils in England can get free school meals during term time up to the end of Year Two but older pupils are only eligible if their parents receive certain benefits.
Families on Universal Credit can only get free school meals if their annual income is less than £7,400 without including benefits.
The cost of living crisis has reignited the row over widening eligibility for the benefit, which has already been committed to in Scotland and Wales.
In a letter to Rishi Sunak and Nadhim Zahawi, teaching unions and campaigners warned that school dinners are sometimes the only hot nutritious meal that children eat a day - and hunger can act as a "real barrier to learning".
The group, which includes bosses of the NEU, ASCL, NASUWT and TeachFirst, said: “A quality school meal helps improve children's concentration and behaviour during lessons. We witness, first-hand, the effect they can have on improving school attendance, on children's health, and academic performance.
"However, the intensifying cost-of-living crisis means many more are now struggling to afford school lunches... We see the devastating reality of children coming to school unable to afford to buy lunch, because their family circumstances means they fall outside the restrictive free school meal eligibility criteria."
The letter added: "Now is the right moment for the Government to commit to an expansion of free school meals, providing a nutritional safety net that supports all children to learn and achieve."
Boris Johnson has previously resisted calls to widen school lunch provision, led by footballer Marcus Rashford who shamed the Government into feeding hungry children during the holidays in the pandemic.
The PM's food tsar Henry Dimbleby also demanded a major expansion the benefit by raising the threshold for household income to £20,000, which would extend free lunches to another 1.1 million children.
Jo Ralling, from the Food Foundation, said: “Government urgently needs to reconsider the threshold used in England for free school meals so more children are protected by this vital safety net.
"Far too many of our most vulnerable families are not receiving the support they need and the situation is now critical with the current cost of living crisis."
Dr Nick Capstick, Chair of the School Food Review Board and head of Drove Primary School in Wiltshire, said more children are coming into school underfed or undernourished.
“We are also seeing more of our pupils having time away from school because of illness and poor oral hygiene caused by inadequate diet," he said.
"Schools are increasingly faced with the need to support and often feed young people whose families can no longer afford the right food at home.”
Azmina Siddique, Policy and Impact Manager at The Children’s Society, said: "The cost of living crisis is hitting low income families hard. Rising food and fuel costs are leaving many families struggling to afford the basic essentials.
“Support for children was notably absent from the Chancellor’s announcements last week. The Children’s Society has long been calling for free school meals to be extended to all families in receipt of UC, which would provide targeted support to around 1.5million more children.
"This would save a family around £400 a year per child, and ensure children are get at least one hot and nutritious meal every day."
A Government spokesperson said: "We recognise that millions of households across the UK are struggling to make their incomes stretch to cover the rising cost of living, which is why, in addition to the over £22 billion announced previously, we are providing over £15 billion in further support, targeted particularly on those with the greatest need.
“We have also expanded access to free school meals more than any other government in recent decades, currently reaching over 1.7 million eligible children.
“The Holiday Activities and Food programme runs during major school holidays, and wider welfare support is available through the Household Support Fund, which helps vulnerable families in need with essentials, such as food and utility bills.”
