Schools in England are failing to diagnose at least 80% of pupils who have dyslexia, according to a report.
It said families were paying up to £1,000 for help and pupils from poorer backgrounds were being left behind.
The British Dyslexia Association said diagnosis and support was the worst it had seen since government funding started in the 1980s.
The government said it was committed to making sure all children with dyslexia achieved well in education.
By law, publicly funded schools and local authorities must try to identify and help assess children suspected of having dyslexia.
Published by the All-Party Parliamentary Group for Dyslexia and other Specific Learning Differences, the report examined the financial and attainment cost to education of students who had not been diagnosed or properly supported.
Out of 8.7 million school children in England, the report estimated about 870,000 of them have dyslexia but fewer than 150,000 were diagnosed, according to Department for Education figures.
It said when young people were "lucky enough" to get a diagnosis, it was "far from given that they will receive specialist support at all or to an adequate level".