Poorer kids in Cornwall are significantly less likely to be ready for school than their wealthier peers, new figures show.
It comes as deprived children across England lag behind their wealthier peers, with a think tank warning the figures are "extremely alarming".
At five years old, children's development is tested across 17 early learning goals, which cover skills such as motor functions, communication and language, mathematics, and personal, social and emotional development.
They are designed to determine whether a child is 'school-ready' at the end of their reception year.
New Department for Education analysis shows around 839 children in Cornwall are in the bottom fifth for deprivation across England.
Of these, 38% did not reach the expected development standard across these areas by the end of the 2023-24 academic year.
However, for Cornwall children in the wealthiest fifth, this fell to 25%.
Of all children in Cornwall, 31% were deemed not ready for school – down from 34% the year before.
Anne Longfield, executive chair of the Centre for Young Lives and a former Children’s Commissioner, said the development gap between disadvantaged children and their peers is extremely alarming.
She added: "This is not only holding back the life chances of many thousands of children, it is bad for our economy, it puts additional strains on schools and teachers, and it is often a warning sign for future school absence and disruption."
She welcomed the Government's focus on improving early years development, including the roll-out of breakfast clubs and new nurseries, but urged ministers to deliver on their pledge.
Purnima Tanuku OBE, chief executive of the National Day Nurseries Association, said more must be done to support deprived children.
She added: "Nurseries are more likely to close in areas of deprivation so the Government must look at funding more resources in those places."
Nationally, 42% of the poorest children did not meet the expected standard last year, compared to 25% of the wealthiest quintile.
Across all children, 34% were not ready for school.
Early Education Minister Stephen Morgan said: "It’s simply not good enough that almost a third of children aren’t school-ready at age five.
"Teachers being taken off teaching to manage issues with school-readiness holds back every child’s learning and life chances.
"This Government has inherited an early years system that fails to drive the highest outcomes and tackle inequality, but we have a clear mission to break down barriers to opportunity and ensure tens of thousands more children are school-ready through our Plan for Change.
"We have already started that work: extending early language support, investing £15 million to deliver the first phase of school-based nurseries, and improving training and guidance for early educators to better support and assess children with special educational needs."