State nurseries in Britain are facing a financial crisis after years of “unjustifiable neglect” and indecision by the government, teachers warn as they prepare to petition the prime minister to “save maintained nursery schools”.

Headteachers, parents and children will gather at Downing Street on Monday to hand in a petition calling for increased state funding for nurseries to secure their long-term future.

Maintained nurseries are run and inspected like schools, with qualified teachers leading a team of early years specialists, but do not have the same funding entitlements as schools and are therefore facing a funding deficit.

The National Education Union (NEU) says these nurseries will lose an estimated third of their budget when short-term funding commitments – a “stopgap” measure started in 2017 – expire in April 2021.

Mary Bousted, NEU joint general secretary, said the lack of clarity about long-term funding meant nurseries were in crisis. “Heads of maintained nurseries are hanging on by their fingernails,” she said.

One of the best ways [the government] can level up is by keeping these nurseries going and well-funded Mary Bousted

She added that the majority of maintained nurseries are “beacons of excellence” in deprived areas, and their closure would hit vulnerable and disadvantaged families. “These are exactly the areas that Boris Johnson and this government talk about in terms of levelling up. Well, one of the best ways they can level up is by keeping these nurseries going and well-funded ... I think it’s an act of unjustifiable neglect.”

Rachel Gillet, head of two maintained nurseries in Warwickshire, said: “I have moments when I’m looking at a deficit I can’t close because I’ve taken all the measures I can to cut costs. I have to sit in the sandpit and watch the children, and remember exactly what I’m fighting for.”

She added that it is difficult to recruit high-quality staff because she can only offer temporary contracts. “It’s frustrating. I’m passionate about early education, and I do wonder why I’m still having to make the argument for it.”

A Department for Education spokesperson said: “We recognise the importance of maintained nursery schools … particularly in disadvantaged areas. We are providing around £60m of supplementary funding during the 2020-21 financial year.”

The government was committed to funding maintained nurseries in the longer term, she said, but declined to comment on how much funding the government will provide beyond the 2020-21 financial year.