Chronically malnourished children struggle to read and write simple sentences regardless of their level of schooling, according to research by Save the Children.
Eight-year-olds who are stunted by malnutrition are 19% more likely to make a mistake while reading a simple sentence, the report Food for Thought said.
Stunted children are 12.5% more likely to make a mistake writing a simple sentence and do 7% worse on basic maths questions.
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.
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"My sincerest apologies to everyone who purchased tickets and were planning to come to the upcoming shows."
His comments at the Defence and Security Forum on Monday came after a £20 billion black hole in the budget for the next decade emerged.
The Foreign Secretary said he thinks the health service should be "at the very top of the list" for a windfall after the UK leaves the EU.