Stephen Kingdom, of the Disabled Children's Partnership which represents charities, campaigners and parents, said the legal right to an individual plan must remain a key part of any reformed SEND system.
"The absence of any real detail on proposed reforms, and the lack of any government reassurance in relation to media stories about scrapping plans, has caused a huge amount of distress to many SEND families who are already struggling.
"The SEND community is not asking for anything special - we are simply fighting for an ordinary education, an ordinary commute to school, and a seat in a classroom where a child or young person can learn," Mr Kingdom said.
The National Audit Office said last year that the SEND system as a whole "wasn't financially sustainable" and was not delivering better outcomes for children and young people, despite big increases in high-needs funding over the last 10 years.
A deal holding £3bn of SEND deficits off local authorities books has just been extended to 2028, but local authorities are still under huge financial pressure because of rising needs.
In response to Thursday's figures, Tim Oliver, chair of the County Councils Network, said "comprehensive reform" was needed and should include "clearly defined support" for families.
"Despite this record expenditure - which importantly is being artificially kept off local authority balance sheets - we recognise that there is widespread dissatisfaction with the system," he said.
"Councils are facing a deluge of requests for support, so whilst the commitment to reform is important, government can ill-afford to get it wrong."
On Thursday, education secretary Bridget Phillipson said "too many children are not having their needs identified at an early enough stage".
She said this creates "a vicious cycle of overwhelmed local services and children's support needs escalating to crisis point".
"Through our Plan for Change, we're improving things right now, and will break this vicious cycle with wide-ranging reform," she added.
"We've already invested £740 million to create more places for children with SEND in mainstream schools, we're increasing early access to speech, language and neurodiversity support, and we'll set out our full plans to improve experiences for every child and family in the autumn."