A chronic shortage of fully qualified teachers in England's private and voluntary nurseries is risking young children's education, warns a charity.
Last year more than a quarter of a million under-fives attended non-state nurseries without a qualified teacher, says a report by Save the Children.
Applications for early years teacher training have plummeted, leaving nurseries struggling, say the authors.
The government said staff quality was already good and continued to improve.
Contact with highly qualified staff is crucial to young children's development, says the report.
The researchers focused on privately run, voluntary and other independent childcare settings as opposed to state-run nurseries, which tend to be attached to primary schools and are more likely to employ qualified teachers.
They looked at three- and four-year-olds, 95% of whom attend some form of childcare each week.
Of the three-year-olds who do, 36% attend state-funded nurseries and 64% private or voluntary settings.
Of the four-year-olds, 80% are in maintained nurseries and 20% in independent childcare.
The charity found that children in independent nurseries without an early years teacher were almost 10% less likely to meet expected levels of development at five than children in nurseries with qualified teachers.
This leaves them struggling with basic skills such as speaking full sentences, using tenses and following simple instructions - and they are likely to remain behind throughout their school lives, with a knock-on effect on their employment prospects, the report says.
Pre-schoolers not in any form of childcare fare worse still and are less than half as likely to reach a good level of development than those in nurseries with qualified teachers, it says.
The report, based on new analysis of official figures, also found that in the academic year to July 2016, half of all three- and four-year-olds or more than 280,000 children, had attended a private, voluntary or independent setting without a teacher who held a degree-level early years qualification working directly with them.
Overall, just half of independent nurseries in England employed a qualified teacher last year, says the report.
But this masked wide variations between local authorities: 86% of young children in Sunderland were in childcare settings with qualified teachers, but only 16% in the London borough of Newham.
And the report quotes National College of Teaching and Leadership figures showing a fall in early years teacher trainee numbers.
In 2013-14, 2,327 candidates were funded to train as early years teachers - but in 2015, there were just 860 applicants.
Save the Children wants the government to invest in a qualified early years teacher for every nursery in England, starting with impoverished areas including Blackpool, Oldham, Birmingham and Barking and Dagenham in east London.
"If the government is serious about creating a country that works for everyone, it's crucial we urgently invest in a qualified teacher for every nursery across the country, giving children the support they need to reach their full potential," said Gareth Jenkins, Save the Children's director of UK poverty.
A Department for Education spokesman said the proportion of full-time staff with the equivalent of at least A-levels had grown from 75% of 87% in the five years to 2013.
"We want to continue to attract quality staff into the early years, including more trained graduates," said the spokesman.
"We are developing a workforce strategy that aims to remove the barriers to attracting, retaining and developing great people, and we will be investing a record £6bn in childcare by the end of this Parliament."