The former children’s commissioner for England has called for free school meals to be extended to all families on universal credit, while acknowledging that poverty has not been tackled in this country “well enough”.

Speaking on BBC Radio 4’s Today Programme, Anne Longfield, who is chairing a year-long commission on young lives, said she had supported the extension of free school meals to all families receiving the benefit for “some time”, adding that this was something Marcus Rashford and Henry Dimbleby had also called for.

It came as teaching unions have written to the chancellor and education secretary asking for free school meals to be provided for all children from families receiving universal credit in England.

In a letter seen by the BBC, unions and organisations claiming to represent 1 million school staff asked for an urgent expansion of the scheme amid the cost of living crisis. It said vulnerable children not receiving the meals were facing a “real barrier to learning”.

“We see the devastating reality of children coming to school unable to afford to buy lunch, because their family circumstances mean they fall outside the restrictive free school meal eligibility criteria,” the letter said.

Longfield said: “[Free school meals for families on universal credit] is something that makes sense in any day. But at the moment we’re in a huge financial turmoil, families are struggling with the huge cost of living increase, so [free school meals] 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.”

Longfield said she supported making schools meals free for all primary-age children, as it “takes away the stigma from the young children which would take free school meals”.

She added: “But ultimately this is about tacking poverty, which is something we haven’t done in this country well enough”.

However, she said a first step would be to extend it to families from lower incomes.

In England, all infant state school pupils up to year two can get free school meals during term time. For pupils in year three and above, those living in households on income-related benefits (such as universal credit) are eligible, as long as their annual household income does not exceed £7,400 after tax. That figure does not include welfare payments.

In Northern Ireland the cap is set at £14,000 a year. Scotland and Wales have committed to providing free school meals for all primary school pupils.

A government spokesperson, in response to the letter, 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 £22bn announced previously, we are providing over £15bn in further support, targeted particularly on those with the greatest need.”