Tens of thousands of ill-prepared primary school pupils lack social skills, have speech problems and poor personal hygiene, a national study has found.
Almost one-third of new starters are not considered to be ready for the classroom, according to primary school leaders.
The State of Education report, by schools leaders service The Key, estimated that at least 194,000 pupils could be starting ill-prepared for the classroom come September.
Lack of social skills (79%), delayed speech (78%) and deficient self-help skills/resilience (69%) are believed to be the most common reasons for children not being at the expected level when they enter school.
More than half of the 1,188 primary school leaders surveyed also said that under-prepared pupils are arriving with reading (58%), writing (56%) and numerical levels (55%) below the standard they would expect.
While some heads said that pupils were arriving without toilet training, others commented on the impact of technology on children being ready for the classroom. One headteacher said four-year-olds "know how to swipe a phone but haven't a clue about conversations".
Speaking about the findings Fergal Roche, The Key chief executive, said: "School leaders are already struggling to retain staff and manage their teachers' workload, so add thousands more pupils arriving ill-prepared for the classroom to the equation, and the burden placed on our schools will be huge.
"To lessen this load more should to be done to ensure children are arriving at school with the skills they need to learn.
"An agreed definition of what 'school-readiness' means, could be the first step to helping schools, parents and early years practitioners identify what national or localised support is required to meet this growing issue."
At secondary school level, the majority of school leaders cited low reading levels (chosen by 76%) as one of the most common reasons for children arriving under-prepared, along with lower than expected standards of writing (63%) and numeracy (56%).
However, fewer pupils joining secondary schools are thought to be below the expected standard than those joining primary schools.
Gareth Jenkins of Save The Children said the report "provides yet further evidence that too many children are not getting the support they need to thrive in their early years".
He said: "Research for Save The Children has shown that falling behind in their early years can drastically limit a child's chances of success later on, affecting results throughout school, and even earnings as adults.
"Yet the statistics show that one in four children and a third of the poorest are starting school without the necessary speech, language and communication skills.
"As well as support for parents, good quality nurseries can make all the difference. We must invest in a high quality early years workforce to avoid condemning thousands of children to a life of lost potential."