The funding crisis in special needs education is deepening, with council overspends on support for children with conditions including autism and attention deficit hyperactivity disorder rising by 30% in just a year, the Observer can reveal.

Figures sourced under the Freedom of Information Act from 118 of the 151 local authorities in England show that councils are expecting to overspend their high needs block budgets by £288m in 2019-20 – up from £232m in 2018-19. When money raided from mainstream schools budgets is included, however, these figures rise to £315m in 2018-19 and nearly £410m this year – a rise of almost 30% in the space of 12 months.

The high needs block is government funding that supports children with higher cost needs. Children with moderate special needs are funded via mainstream schools budgets.

The shadow education secretary, Angela Rayner, said: “The government has slashed funding for schools and now we are seeing the consequences. This overspend reveals the stark reality that our children are not getting the support they need.

“Ministers need to abandon any plans for tax cuts for the rich and invest in our pupils and schools instead – ensuring they have the resources to support every child, particularly those with Send [special educational needs and disabilities].”

The huge council overspends are despite the government injecting £250m of emergency funding over two years into special needs education last December.

The situation may get worse. Last November, the Observer revealed that 117 councils were forecasting a collective high needs overspend of £200m in 2018-19. At that point, Surrey council was expecting to achieve a balanced budget “on the basis that savings would be found”. Instead, it overspent by nearly £15m – the highest in the country. Its response to the Observer’s FOI request shows an overspend of £31m this year. A council spokesperson insisted that there was no forecast overspend this year – but this will only be after the council pumps in £29m to try to balance the books.

Since 2014, councils have overseen an increase of nearly 50% in young people with education, health and care plans Anntoinette Bramble, Local Government Association

Gillian Doherty, who founded Send Action which campaigns for children with special needs, said: “Despite the increase in councils’ so-called ‘overspends’ we continue to see cuts to specialist support This is contributing to a growing attainment gap between disabled and non-disabled pupils.

“There’s no doubt that government underfunding is having a negative effect on the educational outcomes and life chances of disabled children, undermining inclusion and increasingly raising serious safeguarding concerns due to inadequate staffing ratios.”

Most councils have adopted plans to reduce the special needs deficits. This often takes the form of increasing local special needs provision to lessen costly out-of-area placements, but this will take time to have an effect. And some councils are considering direct funding cuts.

Cambridgeshire council will consult on savings proposals this autumn to cut special needs top-up funding for mainstream schools and high needs units, review top-up rates for further education and cut other high needs spending such as after-school clubs.

A Cambridgeshire council spokesperson said: “Despite a new formula, the high needs block funding is not reflective of the current level of need or costs for children in Cambridgeshire with special needs.

“We have significant pressure in delivering specialist services across a large geographic area and we have faced year-on-year increases in both the number of children being supported and the complexity of their needs.”

Anntoinette Bramble of the Local Government Association said: “Councils have seen rapid rises in demand for support following changes in 2014 which extended eligibility to the 16-to-25 age group. [Since then,] councils have overseen an increase of nearly 50% in children and young people with EHC [education, health and care] plans – or, in their previous form, SEN statements.

“Councils are facing a high needs shortfall of up to £1.2bn next year, which we are calling on the government to address in the upcoming spending round.”