Many schoolchildren could be failing reading and writing tests because they are unaware they are dyslexic, new government-funded research suggests.
Campaigners say dyslexia could affect one child in five
A study by Hull University academics of 1,300 children said dyslexia was a major cause of failure.
Over half of those who did not achieve expected levels in SATS tests displayed all the signs of being dyslexic.
The research has led to calls for more specially-trained teachers in schools as well as better diagnosis.
The study, commissioned by dyslexia charity Xtraordinary People, claims to be the first to firmly identify dyslexia as a major cause of underachievement in primary schools.
It looked at seven and 11-year-olds who failed to reach expected levels of reading and writing in national tests.
The majority - 55% - displayed all the signs of being dyslexic.
Campaigners say the research shows many such children are being let down by schools as they could overcome the reading disorder if they received a diagnosis and specialist teaching.
They claim the reading disorder could affect as many as one in five children and that too often it is ignored in the classroom.
The government is piloting a scheme using specially-trained dyslexia teachers, but says it will not roll it out nationally until it is sure the teachers make a difference.
Schools minister Andrew Adonis said the government had funded the research in question, because it wanted to ensure children with learning difficulties got the help they needed.
He added: "The proportion of children learning to read effectively has risen significantly in recent years, and we are investing in better training for teachers in dyslexia and other learning difficulties.
"But we intend to study the results of the pilots carefully with a view to further action."