Headteachers are "dismayed" by the government's "continued refusal" to provide free school meals over the half-term break.
It is "perfectly obvious" that "many more children are at risk of holiday hunger" as a result of the coronavirus crisis, and it is "fantastic news" that businesses, charities and councils have stepped in to help, according to the Association of School and College Leaders (ASCL).
But this "should never have been necessary", and the government must now "swallow its pride, reverse course and guarantee free school meal provision during holiday periods throughout the Covid emergency", the union said.
Free school meals: Labour to push for new vote on extending free school meals
The government is facing increasing pressure to perform a U-turn on free school meals after it voted down a bid to extend them over the holidays until Easter.
Government under pressure over free school meals
More than 2,000 paediatricians have signed a letter urging prime minister Boris Johnson to extend provision to vulnerable children during the holidays – saying childhood hunger should "transcend politics".
And yesterday Labour announced that it will push for a new vote on extending free school meals into the holidays if the government does not back down before Christmas.
Geoff Barton, ASCL general secretary, said: "We are dismayed by the government's continued refusal to extend free school meal provision over the half-term holiday.
"It is perfectly obvious that the Covid emergency has increased both the extent and depth of poverty among families and that many more children are at risk of holiday hunger.
"The fact that we have seen such an outpouring of help from businesses, charities and local authorities over the past few days to fill the void left by the government is fantastic news and we are incredibly grateful to all concerned.
"But this should never have been necessary and the government must swallow its pride, reverse course and guarantee free school meal provision during holiday periods throughout the Covid emergency."
Mr Barton added: "This issue has shone a light on the hardship faced by many families not only now but in normal times. It is shameful that in a wealthy nation likes ours, holiday hunger is a fact of life for far too many children.
"In the longer term, the government must do more to tackle child poverty, and a good starting point would be to extend free school meal provision to holiday periods on a permanent basis."