Headteachers are concerned that if schools are closed over Covid-19, many children who get free school meals will be forced to go without their day's most nutritious meal.
They want to help those kids by providing supermarket food vouchers to families, using the government money provided to them to pay for free school meals.
Schools receive around £2.50 per child per day for those entitled to free school meals, but under current rules they are not allowed to give that money - in voucher form or otherwise - to families in need when schools are closed.
The letter from headteachers to the Department for Education, which has 125 signatures, says if schools close over coronavirus, many children will "miss out on their only hot meal of the day".
Prime Minister Boris Johnson, in his daily coronavirus press conference, appeared to suggest schools will eventually shut and said the education secretary was ready to make sure children did not miss out on food.
He said: "As we come to the decision on schools we will have plans ready to go on that."
"Gavin Williamson the education secretary has a plan to make sure that parents with kids who are eligible for free school meals get the compensation or the treatment they need, one way or another.
"We have certainly anticipated that."
The letter to Mr Williamson said: "As we plan for all eventualities, we are left with uncertainty about what constitutes acceptable use of the free school meal funding we are in receipt of on their behalf.
"A simple solution is to provide their families with vouchers redeemable at all major supermarkets."
It goes on: "We therefore urge the Department for Education to urgently make clear that school leaders who choose to ensure their vulnerable students are provided for in this manner will face no negative consequences, or to communicate what the department’s preferred alternative is."
Stuart Lock, CEO of Advantage Schools, one of the people behind the letter, told ITV News it would be an "easy way" for Education Secretary Gavin Williamson to provide additional support for children.
He says with school closures now seeming "inevitable", headteachers want to be sure they can support the most "underprivileged students" without fear of consequences.
He said schools "very resourceful" and many will be putting on meals for people to collect.
But he said due to government guidance, it may not be advisable for children to collect meals from schools and in some cases supermarket vouchers could be the only way to provide support.
He urged a quick response for the government so schools can start to "plan for all eventualities".
It is understood the issue was raised with Education Secretary Gavin Williamson during a meeting with school unions.
ITV News Political Editor Robert Peston spoke to the deputy governor of a London school, who told his "desperate worry that children on free school meals won’t eat or eat properly if sent home".
He said there is an "urgent" need for the government to provide free meals for children who live in poverty.
“Why isn’t there a government plan for schools?" he asked.
Jim McMahon, Labour MP Oldham West and Royton pointed out that, if households are hit by the virus, many families may be out of work and as a result, unable to afford to pay for out-of-school meals.
He said on Twitter: "This is real concern, especially as we are close to school closures.
"Critically it isn't just access to food, but also the ability to cook if household incomes are hit. As a minimum the Government need to step in and give protection for those on pre-payment meters."