Many children are not ready to start school when they first enter a classroom, according to a survey of head teachers.
The poll, from the National Association of Head Teachers, found many heads thought "school readiness" had become worse in recent years.
Some say most new pupils are not ready for lessons.
The findings come as the new academic year gets under way and many children start their first term at school.
The National Association of Head Teachers (NAHT) said the findings highlighted the need for greater government investment in education, particularly for early years, and more money for family services.
However, ministers have said that school funding is at its highest level ever, and in July, Education Secretary Justine Greening announced an extra £1.3bn for schools over the next two years.
NAHT members in England, Wales and Northern Ireland were invited to take part in the survey in June and July.
Of the 780 who responded, 83% thought there was an issue with school readiness, and of these, 86% thought the issue had worsened in the past five years.
About a quarter (24%) felt most of their intake was not school ready.
Asked to rank the issues causing concern, children's speech, language and communication was found to be causing greatest concern, followed by personal, social and emotional development, including behaviour issues, and physical development.
School leaders were also asked the likely reasons why children were not school ready.
Failure to identify and support children's additional needs was the most popular answer, followed by parents having less resources and pressure on family life, and then reductions in local services to support families.
NAHT general secretary Paul Whiteman said: "We want to see extra money for education, including early education before children start school, and renewed investment in critical services for families.
"Without proper investment, the youngest and most vulnerable in our society will be starting off behind, with uncertain chances of catching up."
The survey was carried out in partnership with the Family and Childcare Trust, whose chief executive, Ellen Broome, said research had repeatedly shown that high quality early years education was crucial to school readiness, particularly among children from low income groups.
"The government must make sure that every child can access high quality early education and that parents can get the right support to help them to give their children the best start in life," said Ms Broome.