It comes as government figures show that only 58 per cent of Education, Health and Care Plans (EHCPs) are being completed within the 20-week time limit.
Furthermore, there has been a 35 per cent increase in the number of requests for EHCP assessments that have been refused by local authorities.
The statistics, published by the Department for Education, show that as of January 2017 there were 175,233 children and young people with statutory EHCPs and 112,057 with statements of SEN.
This combined total of 287,290 is an increase of 30,975 (or 12 per cent) since January 2016 – this continues a rising trend since 2010.
Furthermore, during 2016, 37,751 children and young people were assessed and 36,094 (95.6 per cent) had a new EHCP made.
A further 10,654 children and young people were either still being assessed or were awaiting a decision after assessment as of January 2017.
In 2016, there were 14,795 initial requests for an assessment for an EHCP that were refused, an increase of 3,860 (35.3 per cent) from the 2015 calendar year.
However, of the 36,094 children and young people who received EHCPs in 2016, only 58.6 per cent of these were issued within the 20-week time limit (a decrease from 59.2 per cent in 2015).
This figure falls further to 55.7 per cent when including cases where exceptions to the time limits are allowed under the regulations.
Catriona Moore, education policy officer at the NAS, said: “The figures show that many children and young people are still waiting far too long to get the right education support, with only 58 per cent of EHCPs being completed within the 20-week time limit. Delays spread uncertainty and can be incredibly stressful for families, especially those who are struggling and in desperate need of support.
“It’s worrying to see that there has been a 35 per cent increase in the last year in the number of requests for EHCP assessments refused by local authorities. Without carrying out an assessment, it’s impossible to know what a child’s needs are, and there’s a risk that children are going without the support that would help them succeed at school.
“The National Autistic Society strongly urges the next government to do more to help under pressure local authorities.”
EHCPs were introduced as part of far-reaching SEND reforms in 2014. They are slowly replacing the system of SEN Statements and School Action and cover a young person from age 0 to 25.