Pupils with autism have to wait more than two years for extra help in school - and their families have to "fight a daily battle" for the support, according to new research.
Half of parents questioned by the National Autistic Society said they waited a year to get the "right support" for their child, and 27 per cent said they had waited for more than 24 months.
A total of 18 per cent of the 1,000 families who took part in a survey conducted by the charity said they had to take legal action to get extra help for their child in school.
Half of those questioned said their child's special educational needs (SEN) were not picked up in a timely way. Sixty per cent said this wait meant the pupil's mental health suffered and 68 per cent said it had an adverse impact on their behaviour.
The charity has now launched a campaign, Great Expectations, to persuade the Government to improve support for children with autism. This includes easier and quicker access to specialists and other support.
Representations will be made to children's minister Sarah Teather, who is reforming the SEN education system.
Charity bosses will argue that parents should be provided with more "robust and simple" ways to challenge school decisions, receive better information about services, and be "equal partners" in the SEN system so they can be involved in decisions about their child's education and about planning local services.
National Autistic Society chief executive Mark Lever said: "It is completely unacceptable that so many parents are still fighting a daily battle for their fundamental right to get an education for their child.
"The Government rightly recognises that action is needed, and that they need to reform a system which continues to let many children with autism down. Our report sets out the practical, often simple, steps that the Government can take to create a system that works for everyone."
He added: "The proposed 'biggest SEN reform in 30 years' will shape the future of a generation of children with autism. Let's get it right."
One in four young people with autism who took part in the survey said they were not happy at school, and 21 per cent said they did not feel safe. A third told researchers they only had one friend, or no friends at all.
Researchers also found that teachers were routinely asking parents to take their child home during the school day for reasons other than illness. A third of families questioned said they had been asked to pick up their child early or take them home at lunchtime, and 19 per cent had been asked to do this on more than four occasions.