Delaying a summer-born child's entry to primary school has little impact on attainment, research suggests.
Children born in England between April and August, whose start in Reception was put back a year, did only marginally better in Year 1 tests, according to a government study.
The number of applications to councils for delayed entry has risen sharply.
Headteachers' unions want clearer guidance on whether delayed school entry for summer-born children works.
Department for Education researchers looked at results achieved in the Phonics Screening Check, taken by pupils at the end of Year 1.
Pupils whose school start was delayed a year in 2014 and 2015 scored on average 0.7 marks higher than other summer-born children.
The researchers say the difference is not statistically significant.
Pupils who were not summer-born outperformed both the delayed and normal admission summer-born pupils.
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The analysis also found parents with higher incomes were significantly more likely to request a delayed school start for their child.
It also found the majority of applications, 74%, were from white British families.
Children in England usually start school in September after they turn four but parents of children born between 1 April and 31 August can request to delay entry for a year.
Typically, this would mean a child starting school in Year 1, forfeiting Reception year.
If a parent wishes to delay their child's admission to school until compulsory school age, at five, and be admitted into Reception, a request needs to be made to their local authority, for their child to be admitted out of their normal age group.
Councils are required to make a decision in the child's best interests, taking account of parents' views and information about the child's development.
Pauline Hull, of the Summer Born Campaign which has called for more flexible admissions, welcomed the report but said "the most important comparison is how much better these children are faring having started school at age five, rather than age four, and the DfE research does not address this.
"The focus on phonics testing doesn't consider social and emotional benefits of delayed entry which is of significant importance to parents."
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The Department for Education surveyed 152 local authorities in England about their admissions policy.
Of the 92 who responded, 1,750 requests were made to delay school entry in 2016-17, up 84% on the previous year.
Overall, 75% of requests were granted.
Ten of the local authorities said they had a policy of automatically agreeing on all requests made to delay entry while 23 said they only agreed on requests where parents presented strong evidence.
The majority said they still expected parents to make a case as to why their child should start Reception late.
Some councils said they were concerned that parents would delay entry for their child because it might benefit them in other ways, rather than because there was a genuine need.
"When a parent does not get a place at their preferred school they will delay their child's entry simply to get another chance to get a place at a particular school," said one.
In a separate survey of parents from Liverpool, Hertfordshire, Devon, and Lewisham, who had all delayed their child's entry into Reception, 79% had a household income of £25,000 or more. Almost half earned more than £50,000 annually.
97% feared their child was not ready for school
38% cited a medical condition or developmental delay
13% said a lack of places in their preferred school was a factor
Paul Whiteman, general secretary of the National Association of Head Teachers, said: "It's about taking a sensible and human approach based on the individual needs of the child in question.
"The flexibility required in order to accommodate the needs of some children born between April and August each year can cause some organizational and financial issues for schools but they are not insurmountable and more guidance from the government has certainly helped."
Julie McCulloch, Director of Policy at the Association of School and College Leaders, called for "a consistent approach across the country" and urged the DfE to "review this matter to develop a policy based on emerging evidence over the impact of delayed entry to school and what works best for children".
A DfE spokeswoman said, "the phonics check measures a specific skill and doesn't tell us anything about a child's development more widely".
"We know the Reception year is an important part of school life and we want to make sure no child misses that year of their education because they have summer birthdays."
