Hundreds of nursery school heads and governors will march on Downing Street on Monday to deliver a letter to Philip Hammond asking for long-term funding to secure their future.

The 251 signatories to the letter will ask the chancellor to safeguard the future of maintained nursery schools in the forthcoming spending review.

The march and letter have been coordinated by the all-party parliamentary group for nursery schools, nursery and reception classes, and the National Association of Head Teachers union.

The children’s minister, Nadhim Zahawi, announced £24m of additional funding for nurseries in February, but the letter argues that there is no guarantee of adequate funding after the next academic year.

“The long-term survival of maintained nursery schools still hangs by a thread,” it reads. “The fact remains that, even with the supplementary funding, most maintained nursery schools have had to make large cuts and make hard decisions to balance reduced budgets.

“Exceptional, highly trained early-years staff have been lost to the system through restructures or because of the constant worry of an uncertain future.

“As headteachers and governors we are trying to plan for a future that, without the sustainable funding, will probably mean the closure of our schools.”

It concludes: “There will be a terrible cost to our social fabric, and the wider education and care system, if our schools cease to exist.”

The march takes place against the backdrop of widespread concern at the effects of cuts across education.

On Friday, a letter from headteachers at more than 7,000 schools was sent to millions of families across England, accusing the Department for Education of refusing to recognise the impact of cuts.

A Guardian investigation found that schools across the country were resorting to cost-saving measures that included cutting the start and end of the day, and making teachers take on cleaning duties and turn off heating.

The Observer revealed in October last year that half of all nurseries, preschools and childminders in England were receiving less government funding than they had been five years previously.

A Conservative manifesto pledge to allow working parents of three- and four-year-olds to claim 30 hours of childcare a week during term-time for free has also led to more than half of private nurseries raising their fees.

Zahawi said: “We know that maintained nursery schools play a valuable role in supporting some of the most disadvantaged children across the country, and that there was some uncertainty about funding for the next academic year.

“That is why we have provided an additional £24m of funding to local authorities for their maintained nursery schools – to give reassurance in time for the allocation of places for September 2019.

“Decisions on what happens past the end of the 2020/21 academic year will be taken at the next spending review.”