Budget watchdog the Office for Budget Responsibility warned the change “would imply a 4.0% real fall in mainstream school funding per pupil rather than the 0.5% real increase planned by the Government”.

At a post-Budget press event, Ms Reeves insisted the reforms were “not about money” but about “creating a system that actually works for kids”. She said ministers had already “put in more money into the schools budget in the spending review just before the summer”, and confirmed an ongoing review of SEND provision.

But school leaders warned that any new SEND costs “must not come at the expense” of classroom budgets already under severe pressure.

Paul Whiteman, General Secretary of the National Association of Headteachers, told the Daily Express the “broken SEND system needs significant additional investment” so it can consistently support all pupils. But he cautioned: “Investment must not come at the expense of already-strained core school budgets especially given the government’s desire for more children with additional needs to be educated in mainstream settings.”

He added it was “vital” that reforms “ensure that all parties involved in delivering support, including not only schools, but also councils and health services, are better equipped to do so – ending the mismatch between children’s needs and resources available to meet them”.

Councils say they are already carrying huge SEND deficits after years of rising need, soaring transport costs and legal duties to fund Education, Health and Care Plans.

Cllr Amanda Hopgood, Chair of the Local Government Association’s Children, Young People and Families Committee, warned that existing shortfalls were “pushing many councils to the financial brink”.

She said: “While it is positive the government has committed to absorbing the costs of SEND spending from 2028/29 – and we look forward to clarity on how this will be funded,” the plans still fail to address the mounting deficits already held by local authorities.

She urged ministers to use the provisional Local Government Finance Settlement to “write off these deficits – both the current accumulated deficits and any future deficits expected up to and including 2028/29 – and to fully fund all associated costs such as home to school transport over this period”.

The National Education Union also issued a stark warning, saying the schools system was “running on empty”.

Daniel Kebede, the General Secretary, told the Express: “The crisis in funding is deep. Schools and colleges cannot afford to get into even greater funding difficulties”.

Mr Kebede added: “We hope that the OBR’s fears are wrong and that the reforms to SEND expected in the new year will bring clarity on how the new spending will be handled and not eat into the core schools budget. It would be a grave mistake for the government to pursue this path.”

Stephen Kingdom, campaign manager from the Disabled Children’s Partnership, said: “Last week the Prime Minister said SEND is the subject he is asked most about, this week the Treasury has left parents, young people and campaigners deeply concerned about the future for disabled children’s education. The Budget has created more questions than it answers. How will Government departments absorb the projected £6bn costs of taking financial responsibility away from councils? If this comes out of school budgets, their policy of making mainstream schools more inclusive will fail at the first hurdle. Disabled children will bear the brunt of cost cutting yet again.”