Four out of 10 of England's poorest boys start school without the language skills needed to learn, a study says.
Researchers for the "Read On. Get On." campaign warned that these children, who lag 15 months behind their peers at the age of five, might never catch up.
It found the poorest girls did not fare as badly as boys, but 27% were below the basic language level at that stage.
A Department for Education spokesperson said: "Raising reading standards is a key part of our plan for education."
They added: "In order to raise the quality of early years learning we have improved early years' qualifications and encouraged high-quality entrants to the profession, through Teach First in the Early Years."
Research body Education Datalab analysed the National Pupil Database and data from a longitudinal study of 19,000 children to see how many children in England were reaching the expected level in language at age five.
They also looked at last year's national results of the Early Years Foundation Stage - an assessment of children's ability carried out in schools at the end of Reception year.
Researchers used eligibility as a definition for the poorer children as it compared their language skills on starting school, and their subsequent attainment, with that of their classmates.
The report said toddlers' early language skills were a vital stepping stone in learning to read.
It added that those falling as much as 15 months behind before they reach school are unlikely ever to catch up.
Average language skills for five-year-olds
- Able to understand and talk with new people using well-formed sentences.
- Ask lots of "why" questions
- Able to understand longer and more complicated sentences
- Able to understand and use most everyday words that adults use
- Explain what has happened, and why, in an interesting way
The Read On, Get On campaign is calling for urgent government investment to boost skills of nursery staff to improve language development before school starts.
Dame Julia Cleverdon, chairwoman of Read On, Get On, said: "Poor children, and poor boys in particular, are being set up to fail because too many haven't developed the building blocks of learning before they arrive at the school gate for the first time.
"The government has made a strong commitment on literacy by setting clear goals to get all children reading well by the age of 11. What this research tells us is that this target is at risk unless we close the language gap."
Gareth Jenkins, director of UK Poverty at Save the Children, which backs the campaign, said: "To change the story for the poorest children, we need urgent investment to boost the skills of early years staff and ensure every nursery is led by an early years teacher.
"Only then can we give every child the foundation they need to read and succeed at school and in life."