Teaching organisations have written to the prime minister, asking for all children in households on universal credit to be given free school meals.
It adds momentum to a campaign backed by Jamie Oliver, who has told BBC News the rule change would help some 800,000 of the "most vulnerable" children.
The government said it had already expanded access to free school meals more than any other in recent decades.
It comes as government departments prepare for spending cuts.
The letter, seen exclusively by BBC News, is signed by the leaders of 12 unions and other groups claiming to represent a million teaching staff, governors and school trustees across the UK.
It warns the prime minister, the chancellor and the education secretary that "hunger is now a real issue in our schools" and "too many families" not currently eligible for free school meals are struggling to afford them.
