Children in England with special educational needs and disabilities (Send) have lost out on £1.2bn worth of services because government funding has failed to keep pace with soaring demand for additional support over the past four years, according to an analysis.

The number of children and young people with an education, health and care plan, a legal document detailing a child’s entitlement to support for special needs, has risen from 240,000 to 320,000 since 2015 – an increase of 33% – according to research by the National Education Union (NEU).

Yet central government funding paid into the “high-needs block” of councils’ education budgets to cover Send provision has only increased by 7% over the same period, from £5.6bn to £6bn in today’s prices, the NEU says, resulting in “massive” funding shortfalls in nine out of 10 local authorities.

As a result, families have endured increasing waiting times for Send assessments and cuts to specialist provision and support staff. In response, families have turned to tribunals to fight councils for their children’s rights to additional provision and are winning in the vast majority of cases.

A number of parents have also taken local authorities to the high court over Send funding decisions, and in June the government will find itself in court when its Send funding policy will be examined in a landmark judicial review.

The crisis in Send provision will be discussed during the NEU’s annual conference, which opens in Liverpool on Monday. Kevin Courtney, the union’s joint general secretary, said: “The funding shortfall for Send provision comes against the backdrop of the swingeing cuts to local authority budgets imposed by the Westminster government over the last nine years, which have left many councils on the brink.”

Courtney said that between 2010 and 2020, councils will have lost almost 60p out of every £1 the government once provided for Send services. “This is an appalling way to be addressing the education of some of our most vulnerable children and young people and is causing untold misery and worry for thousands of families.”

One of the key reasons for increased demand, and cost, is the extension of education, health and care plan provision to include young people with additional needs aged 19-25. Campaigners also highlight the drift of Send pupils away from mainstream schools to more expensive special school settings, accusing some of failing to be inclusive, either because of funding or accountability pressures.

Angela Rayner, the shadow education secretary, said: “Education is a right, not a privilege, and these cuts mean that in one of the richest countries in the world children with special educational needs are not receiving the support they need.”

Nadhim Zahawi, the minister for children and families, said it was wrong to imply that funding had been cut and that the government had increased spending on high needs from £5bn in 2013 to £6.3bn this year.

“We recognise the challenges facing local authorities and in December provided an extra £250m up to 2020 to help them manage high-needs cost pressures. We have also provided councils with an extra £100m funding to create more Send places in mainstream schools, colleges and special schools.”