A survey of 780 head teachers conducted by the Family and Childcare Trust and the National Association of Head Teachers (NAHT) found that 83 per cent were concerned with the school readiness of some pupils starting school.
The research highlighted particular concerns about the speech, language and communication skills of pupils with nearly all respondents (97 per cent) reporting it as an issue.
In addition, two-thirds of school leaders blamed children's lack of school readiness on the failure to identify and support additional needs before they started formal education.
Other factors holding children back included pressure on family life and resources (66 per cent), as well as a reduction in local family support services (63 per cent), the report said.
The School Ready report also found that many schools have insufficient resources to meet the needs of children who require additional support - 88 per cent said inadequate school funding was a barrier to improving school readiness, 84 per cent reported a shortage of adult support and 65 per cent were unable to access specialist services.
Paul Whiteman, NAHT general secretary, called for more investment in helping children transition to school.
"We want to see extra money for education, including early education before children start school, and renewed investment in critical services for families," he said.
"Without proper investment, the youngest and most vulnerable in our society will be starting off behind, with uncertain chances of catching up."
Early years groups also said the government must boost funding for childcare providers to better prepare children with additional needs for school.
Purnima Tanuku, chief executive of National Day Nurseries Association, said: "This report highlights the gaps in investment for children who need additional support.
"Children who attend high-quality early years education develop good social and communication skills," she said.
"Early years practitioners are best placed to identify those children at an early age who need extra support to give them the best start. Unfortunately, there is often a delay in acquiring this additional support, which means that many children are not helped until they start school. By this time, the gap in development is much harder to bridge."
Neil Leitch, chief executive of the Pre-school Learning Alliance, added: "We firmly believe that children develop at their own pace and that policy focus should be on ensuring that schools are ready for children, and not the other way around.
"Key to all this, as always, is investment. The fact that such a significant majority of respondents reported that they have neither the funding nor the resources to give adequate support to those children who most need it simply isn't acceptable."
The Local Government Association (LGA) warned that funding cuts for children's services was making it harder for councils to fund early support for children that need it.
Richard Watts, chair of the LGA's children and young people board, said: "Councils have worked hard to protect funding for children's services in response to rapidly rising demand, but ongoing cuts to local authority budgets are forcing many areas to make extremely difficult decisions about how to allocate increasingly scarce resources."
To read more about school readiness, see CYP Now's special report on early learning and school readiness.