Hundreds of nursery schools could be forced to close if the Government goes ahead with plans to offer parents 30 hours of free childcare, a teaching union has warned.

Pre-schools in England would "cease to be financially viable" after the planned two years of financial support from the taxpayer ends, because of the costs associated with the extra places, the NAHT said.

The school leaders' union said analysis carried out with the charity Early Education showed nurseries in the local authority areas with the highest number of nurseries - including Hertfordshire - would see huge cuts in funding if the proposal to double the current 15 hours of free time was introduced.

Hertfordshire has been named as one of eight local authorities that will pilot the scheme, and parents experiences in Watford will be used to support the expansion of the scheme across England in 2017.

READ Working parents in Watford to benefit from more free extra childcare

READ Details of pilot 30-hour free childcare plan for working parents revealed

Its general secretary, Russell Hobby, contrasted the situation with Theresa May's announcement to open a new wave of grammar schools, saying: "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."

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.

In April, a poll of childcare providers carried out by the Pre-school Learning Alliance found that almost half of childcare providers feared they could be forced to close as a result of the 30-hour offer.

The online poll also found that 48 per cent of providers felt they would have to reduce the number of places they offered to other age groups.