Teaching unions have written to the Chancellor and Education Secretary asking for free school meals to be provided to all children from families receiving Universal Credit in England.

Unions and organisations claiming to represent one million school staff asked for an “urgent” expansion of the scheme amid the cost-of-living crisis.

It said vulnerable children who do not receive free meals are facing a “real barrier to learning”.

“Every school day we see the benefits free school meals provide to those currently entitled. For many it is the only hot, nutritious meal they have in a day,” the letter to Rishi Sunak and Nadhim Zahawi 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.”

Signatories to the letter included Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), Dr Mary Bousted and Kevin Courtney, joint general secretaries of the National Education Union, and Dr Nicholas Capstick, chief executive of White Horse Federation Trust and chairman of the School Food Review Working Group.

On BBC Breakfast on Tuesday morning, Dr Nick Capstick, headteacher of Drove Primary School in Wiltshire, said schools have found that “this is an increasingly difficult situation”, with “more and more children… coming in having not had a good meal the night before maybe, and even not having breakfast sometimes.

“And this impacts on the way they perform in school, and also their wellbeing and their happiness, so it really is a major problem in schools and growing.”

Dr Capstick said his staff are seeing lethargic and irritable children with poor concentration who cannot focus on their learning.

He said he would like to see co-operation with the Government for all primary school-aged children to access free school meals, describing the current threshold for eligibility – an income below £7,400 per year – as a “ridiculously low figure”.

He said more funding for free school meals would save money spent on childhood obesity and tooth decay, as well as pupils being absent through illness, meaning their parents stay away from work.

Speaking to the BBC Today programme, former children’s commissioner Anne Longfield said she had supported extending free school meals to all families receiving universal credit for “some time”, adding that the idea was also backed by Marcus Rashford and Henry Dimbleby.

“So it’s something that makes very much sense in any day but at the moment clearly we’re in a huge financial turmoil – families are struggling with a huge cost of living increase so this adds certainty.

“It means that all children get a nutritious meal. They learn and concentrate better, and it makes a positive difference to their physical health and their time in school but ultimately this is about tackling poverty, something that we haven’t done in this country well enough.”

Ms Longfield said she would personally make free school meals available to all primary school-aged children as this would take away the “stigma” of free school meals.

A Government spokesman told the BBC: “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.

“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.”

Mr Barton, of the ASCL, said: “We are very pleased to support the Food Foundation letter calling for free school meals to be offered to all children in families receiving Universal Credit or equivalent benefits.

“It is vital in general that free school meal provision is extended to more children but particularly so at this time because of the cost-of-living crisis which is pushing many families deeper into poverty.

“About one in five pupils in England are currently eligible for free school meals but we know that a larger proportion of children are classified as living in poverty – about 30% – so a significant number of children in difficult circumstances are clearly missing out.

“Ensuring that all children who need this provision receive a free school meal is hugely important, not only in terms of health but also education as children who are hungry are not in a fit state to learn.”

Dr Patrick Roach, general secretary of the NASUWT, said: “Expanding entitlement to free school meals would have significant benefits in terms of pupils’ health, wellbeing and educational attainment at a time when many families are finding it increasingly difficult to afford to put food on the table.

“Teachers see the effects of hunger in the classroom every day and know first-hand how it inhibits children’s ability to concentrate, focus on their learning and make progress.”

Leora Cruddas, chief executive of the Confederation of School Trusts, said that by expanding free school meal eligibility criteria to children in receipt of universal credit “the Government would be providing a safety net to the many children who live in households that missed meals during April”.

“In the current fiscal environment where families are facing huge rises in the cost of living, it is essential to the health and wellbeing of our children that we are able to provide this safety net,” she said.