Dozens of special schools in Hampshire were over capacity last academic year, new figures show.
Data from the Department for Education shows nearly two-thirds of special schools were full or overcrowded across England – with a union warning a lack of funding is letting down children.
The figures show 24 of the 28 special schools in Hampshire were over capacity in the 2022-23 school year.
Across England, 63% of 1,077 special schools had at least met capacity, with around 4,000 more pupils than places.
The Department for Education said this "may be a result of the way capacity has been measured, which does not take account of type of need."
Rob Williams, senior policy adviser at school leaders’ union NAHT, said the figures represent a "complete mismatch" between the needs of these children and the funding available to schools and local authorities.
He said: "There are simply not enough special schools for pupils who need more specialist support.
"Many children are therefore instead being placed in mainstream schools which may themselves be over capacity, and may not have the staff, expertise or resources needed to offer the best possible education and support."
Mr Williams urged increased funding for children with special educational needs.
Earlier this month, Education Secretary Gillian Keegan admitted to school and college leaders the Government had not done enough for these children.
Speaking to the Association of School and College Leaders annual conference, Ms Keegan said: "If you look at special educational needs, we haven’t built enough special educational needs places or schools."
Addressing the same conference, shadow education secretary Bridget Phillipson said the challenge facing children with special educational needs or a disability was "enormous", and it would take time for Labour to "turn that around".
The figures further show nearly a quarter of state secondary schools across England were full or overcrowded, largely unchanged from a year before.
In Hampshire, 24 of 69 schools were at or above capacity.
Meanwhile, 17% of primary schools across the country were facing the same issue – with 92 of them in Hampshire.
In the Spring Budget the Chancellor promised £105 million over the next four years to build more than a dozen new special free schools.
The Treasury said it would create more than 2,000 additional places for children with Send in England.