Hundreds of nursery schools in England could close if the government presses ahead with plans to offer 30 hours a week of free childcare, a union warns.
Pre-schools will not cope financially after a planned two-year package of taxpayer support ends, says the National Association of Head Teachers.
In 2017, free care for three-and four-year-olds is set to rise from 15 to 30 hours a week in term-time.
Ministers say the extra funding will provide stability for nursery schools.
There are 400 maintained nursery schools in England, offering an early years education to three- and four-year-olds.
Two-thirds are in the most deprived areas of England, and 99% are rated as good or outstanding.
The NAHT carried out analysis with the charity Early Education.
They found those in the local authority areas with the highest number of nurseries - including Birmingham, Lancashire and Hertfordshire - would see huge cuts in funding if the proposal to double the current 15 hours of free time was introduced.
- nursery schools in Birmingham received an average of £8.36 an hour in 2015-16, which would fall to £4.44 an hour in 2017-18 - if no additional funding were made available - a decrease of £3.92
- nursery schools in Lancashire received an average of £7.89 an hour in 2015-16, which would fall to £4.27 an hour, if no additional funding were received - a decrease of £3.62
- nursery schools in Hertfordshire received an average of £7.47 an hour in 2015-16, which would fall to £5.36, if not subsidised - a decrease of £2.11
- and nursery schools in Durham received an average of £6.67 an hour in 2015-16, which would fall to £4.15 an hour - a decrease of £2.52 - if no additional funds were injected
NAHT general secretary Russell Hobby said: "The Department for Education's [DfE] own data shows that any funding approach that does not reflect these costs on an ongoing basis will be a body blow for early years education in nursery schools.
"There is additional funding available, but only for two years.
"After that point, England's nursery schools will cease to be financially viable.
"The 30 hours offer will be doomed before it even gets started, additional places won't materialise and current places will be lost as nursery schools across England close their doors for good.
"The government has the data - it must rethink before thousands of families, many in the poorest areas of the country, are left high and dry.
"In contrast to grammar schools, high-quality nursery education is a proven method of helping the most disadvantaged families.
"It is inexplicable that a government serious about social mobility would focus on one at the expense of the other."
The NAHT claims come on the final day of a government consultation on early years funding and the 30 hours of free care in term time.
A spokeswoman for the DfE said the proposals for supplementary funding would be for at least two years.
"This extra funding will provide stability for nursery schools, which make a valuable contribution to improving the lives of some of our most disadvantaged children.
"The funding is part of our record investment in early years - £6bn per year by 2020.
"We will be consulting with the maintained nursery schools' sector on future funding in due course."
Valerie Daniel, head of Washwood Heath Nursery School, in Birmingham, said: "The government seems to recognise the quality of early years education such settings provide, but has no plans to secure their future beyond the two years of transitional funding set out.
"This will lead to a massive loss for nursery schools, with larger settings potentially losing more than £200,000 from their current budgets, which have already been hit by local budget cuts.
"I fear that a significant loss to maintained nursery budgets will create a ripple effect on safeguarding the most vulnerable children in the region."
In April, a poll of childcare providers carried out by the Pre-school Learning Alliance found almost feared they could have to close as a result of the 30-hour offer.
The online poll also found that 48% of the providers felt they would have to reduce the number of places they offered to other age groups.