Chronically malnourished children struggle to read and write simple sentences regardless of their level of schooling, according to research by Save the Children.
A report by the charity, Food for Thought, shows that eight-year-olds who are stunted by malnutrition are 19% more likely to make a mistake reading simple sentences like "The sun is hot".
Stunted children are 12.5% more likely to make a mistake writing a simple sentence and do 7% worse on basic maths questions.
Save the Children chief executive Justin Forsyth said: "These findings confirm our very worst fears - that poor nutrition is capable of seriously damaging a child's life chances before he or she even sets foot in a classroom.
"We have made huge progress in tackling child deaths, but having a quarter of the world's children at risk of under-performing at school will have grave consequences for the fight to end global poverty.
"World leaders must take the opportunity to change this in London on June 8 and commit to tackle the scourge of malnutrition for good. We want to see funding for countries suffering the highest burden so that millions of children's lives can be transformed."
The findings, based on research with thousands of children in Ethiopia, India, Peru and Vietnam, come 10 days before a global nutrition summit in London and ahead of this year's G8. The global economic impact of malnutrition could be up to £80 billion, according to Save the Children.
Shambel, a 12-year-old from Ethiopia, said: "Those children who come to school after having their breakfast do well. This is difficult for me as I don't get enough to eat." Gatluak, a 10-year-old from South Sudan, said: "When I was going to school I used to struggle with lessons because I had often gone without any food."
One in four children worldwide are believed to be stunted, and Save the Children's research points to a literacy and numeracy crisis in the developing world. The report also highlights the huge economic cost of chronic malnutrition as stunted children could earn as much as 20% less in adulthood.
Despite being one of the most cost-effective forms of development assistance, spending on nutrition programmes is just 0.3% of global development spending. Save the Children is urging the public to sign the "Enough Food for Everyone IF" campaign which will hold a mass rally in Hyde Park on the day of the summit.