Schools are serving young children smaller lunches or slashing the amount of meat in sausages and ham sandwiches due to spiralling costs, MPs have been told.
Tory MP Steve Brine sounded the alarm over the funding shortfall for infant free school meals, which has meant schools are left to make up the gap between Government cash and the cost of school dinners.
All children in England are eligible for free school meals up to the end of Year 2, regardless of household income. Older children only qualify if their parents receive certain benefits.
But schools are reporting that the cash they receive doesn't cover the rising costs of food, energy and transport for catering firms.
One school in Mr Brine's Winchester constituency pays £2.80 per school meal, while receiving £2.41 in funding - a gap of 39p per meal.
This is an extra cost of approximately £3,000 a year to the school's budget.
Speaking in a Westminster Hall debate, the Tory Health Committee chairman said: "Put simply there is a gap between the funding received and the cost of putting good quality food on the school table.
"The inevitable impact of this falls on school budgets, which are making up the shortfall."
Mr Brine said the Government had increased the funding rate by 7p per pupil but the truth is this remains "well behind the current rise in food inflation", which is running at nearly 20%.
Some pupils are getting smaller, poorer quality lunches, he said.
"In some cases schools have opted to offer only packed lunches because of the cost of energy of producing them," he said.
"Some wholesalers have reported they are reducing portion sizes, thinner sliced ham in the baguettes and reduced meat content in sausages as two examples. That should worry all of us."
Some schools have even asked parents for a donation towards their child's meal, he said.
He said: "For some schoolchildren, the school meal may be their only hot meal that day. It may be their own meal that day.
"We know the provision of good quality food is key to pupils wellbeing and ensuring they are fully able to engage in their teaching and learning."
He warned school budgets are under pressure and added: "We have to ensure the provision of a good quality meal does not have to be subsidised through funds intended to support core education.
"It's therefore essential that the rate is updated to reflect rising costs."
Labour MP Sharon Hodgson cited research that showed funding would stand at £2.87 per child per meal if it had risen in line with inflation, rather than £2.41 - a shortfall of £150 per year per child.
"The Government is yet again asking schools to do more with less," she said and argued funding should be "fit for the future".
Funding for infant school meals has only risen by 11p since 2014, leaving a "yawning gap", said Lib Dem MP Munira Wilson.
"Schools shouldn't have to choose between cutting and scrimping on teaching budgets and other budgets that benefit children and eroding food standards," she told the minister.
Schools Minister Nick Gibb said: "The rate of funding for universal infant free school meals is regularly reviewed, and I can assure (Mr Brine) and all other members taking part in this debate that I am actively looking at this area."
He added: "I understand and acknowledge the pressures the industry is facing and we will continue to take this into account when determining spending priorities.
"I am confident the offer we have in place through universal infant school meals ensures that those children receive the best start to their time in school."
The Mirror is campaigning alongside the National Education Union (NEU) to extend free school meals to all primary school children.
