Pre-schools in England would "cease to be financially viable" if the government offers families 30 hours of free childcare, a teaching union has warned.
Union for school leaders, NAHT, said the proposal to double the current 15 hours of free time and cease financial support from the taxpayer after two years will hit nursery school providers hard.
Analysis carried out with the charity Early Education showed nurseries in areas with the highest number of pre-schools, including Birmingham, Lancashire and Hertfordshire, would be hardest hit.
In April, a survey showed some 750 providers feared they would have to close as the move would have a detrimental effect on their business.
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."
Mr Hobby added that the plans are in contrast with Theresa May's announcement to open a new wave of grammar schools.
"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," he said.
Thursday is the deadline for submissions to the government's consultation on the plan to extend early years funding for three- and four-year-old children.
The DfE said additional funding was available for nurseries for at least two years and it planned to speak with the industry over funding after that.
But Valerie Daniel, headteacher of Washwood Heath Nursery School in Birmingham said beyond the transitional funding, the government has "no plans to secure their future".
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.
Our proposals for supplementary funding, which takes account of maintained nursery schools' current funding rates, are for at least two years.
The government are brick by brick dismantling many of the educational achievements of the coalition government. Evidence shows that solid progress in the nursery sector does more for social mobility than selection at 11 but yet again ideology trumps evidence.