Children’s services from Sure Start to schools and NHS mental health are at breaking point, according to a coalition of 120 organisations that have called on the chancellor to invest in young people in the budget next week.

An open letter to Philip Hammond and Theresa May from a group of charities, teaching unions and medical colleges accuses the government of ignoring children and young people in its spending plans.

Local authorities backed the call, saying council-run children’s services were fast approaching a tipping point as they struggled to maintain services in the face of a funding gap estimated to reach £3bn by 2025.

“If austerity is really coming to an end, its high time the government puts its money where its mouth is, and makes a concrete financial commitment to the welfare of children,” said Anna Feuchtwang, the chair of the End Child Poverty alliance.

The call comes at a time of increasing concern about rising child poverty, long waits for youth mental health services, school budget cuts and growing demand for services for youngsters with special educational needs and disabilities.

Councils have said financial pressures caused by rising demand for children’s services threaten to tip town halls into bankruptcy. More than 90 children a day are taken into care in England, while family support services undergo massive cuts.

The letter cites a survey by the National Association of Head Teachers that found almost three-quarters of school leaders expected they would be unable to balance their budgets in the next financial year.

Earlier this month the National Audit Office said mental health services for children would fall well short of meeting growing need, despite promises of extra funding, in part because of staff shortages.

Feuchtwang said: “Things we once took for granted, like family support, children’s centres and respite care for families with disabled children are now the privilege of the few. In some areas of the country, over half the children are growing up in poverty.

“For these children and the many others who need urgent help, the services, benefits and support that could provide a lifeline have been cut to the bone. We are failing our children if we don’t put them at the heart of government spending.”

Anntoinette Bramble, chair of the Local Government Association’s children and young people board, said: “This is a compelling demonstration of the grave concerns shared across the sector as funding cuts increasingly leave services struggling to provide the care and support that thousands of children and families rely on.”