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.
Head teachers' 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.