More than a third of summer-born children are struggling with key skills including communication and basic maths and literacy at age five, official figures suggest.
Youngsters born in the summer months are less likely to be considered to have a "good level of development" in core areas than those born in the autumn, at the start of the academic year, according to government data.
Early years experts said there is a "wealth of evidence" that a child's month of birth has an impact on academic grades and sporting achievements.
However, unions warned that testing children at the age of four to provide a baseline for their development distressed children and caused more problems than it solved.
The latest Department for Education (DfE) data shows that in 2019, 62 per cent of children in England born between May and August had a "good level of development" based on teacher assessment at the end of Reception - the first year of school - meaning 38 per cent did not reach this level.
A "good level of development" means they were reaching the level expected of them in their communication and language skills, physical development, personal, social and emotional development, literacy and maths.
In comparison, 81 per cent of their classmates born between September and December had a good level of development - a 19 percentage point gap.
The statistics also show that 61 per cent of summer-born children were achieving the expected level in all the early learning goals, compared with 79 per cent of those born in the autumn (a gap of 18 percentage points).
There are 17 early learning goals in total, which come under seven broad areas. Along with the five areas included in the "good level of achievement", these are understanding the world and expressive arts and design.
Michael Freeston, director of quality improvement at the Early Years Alliance, said: "The attainment gap between summer-born children and their peers is very real. Beyond these statistics, there is a wealth of evidence that the month you were born can impact the grade you get and your sporting prowess.
"The reason these children struggle is simple enough. If you're just four years old when you start school, you could be up to 20 per cent younger than your peers.
"That's 20 per cent less life experience and 20 per cent less time to develop physically and emotionally. Perhaps most crucially, it's a lot less time in early education - it makes it almost inevitable that a gap will develop."
Wakefield-based Sally Kincaid, district secretary for the National Education Union, agreed, adding that the picture was about the same in Yorkshire as the rest of the UK.
She added: “Development in early years is extremely fast. Some of the children are being tested at four years old, when they’re really just babies.”
She said the union opposed testing at such a young age as it often caused “distress” and did not give useful results.
“Countries like Finland with world-leading education systems don’t start until the children are seven. Any early years specialist knows play is the most important thing for children’s development up until that age,” she added.
Pauline Hull, leader of the Summer Born campaign group, said: "The results of these tests are showing us what we know, and actually demonstrate that yes, if they have more time to physically and cognitively develop, then they would be better ready for school, and that is something no pre-school, no parent, no system can accelerate.
"It's very often a natural, physical, mental development that needs to just happen with time."
A Department for Education spokesman said it is "perfectly normal to see younger children performing less well in early years", adding that evidence shows these children make faster progress, with the gap narrowing as youngsters move up through primary school.
"We have given schools and councils clear advice on how to support parents who want to delay their child's admission to reception until age five, to ensure decisions are made in every child's best interests, and we remain committed to amending the School Admissions Code as soon as possible," the spokesman said.