Thousands of UK teenagers cannot read well enough to understand their GCSE exam papers, a large-scale analysis of pupils' reading ability suggests.
Data on 29,000 teenagers in 1,100 schools in England suggests they have an average reading age of 10 or 11.
But GCSE materials and papers were found to be pitched at the correct levels.
The tests were based on results of both struggling and bright pupils using Renaissance Learning software.
Children's literacy levels were checked by asking them 25 questions which required them to put words into a particular context. The results were then combined with teacher assessments.
The researchers also checked six randomly selected GCSE exam papers to determine the average reading age required to comprehend the texts.
The IT firm admits the data on the group is not nationally representative, but says it was alarmed by the results.
Its findings, based on the 29,000 children using its software, suggest 15 and 16-year-olds in England have an average reading age five years lower than their actual age.
This is surprising because both primary school and secondary school results have been rising year on year. Nearly nine out of 10 children in England are deemed to have met the required levels in reading at age 11.
And nearly seven out of 10 GCSE grades are awarded an A* to C.
But James Bell, director of professional services at Renaissance Learning, said he did not believe the data was highlighting the literacy levels of poorer readers.
He said: "There may be a little bit of skewing, but there is no indication that schools are buying it as an intervention programme."
He said schools tended to use the software as a whole school reading programme.
'Practise or regress'
He suggested that although children may learn to read well using phonics in primary school, many did not practise the skill in a formal way once they reached secondary school.
He said: "Other research that we have done has shown that if children are reading and understanding on a daily basis for 19 to 24 minutes a day there's substantial growth.
"If they are reading and understanding for less than three minutes a day, they regress."
Jonathan Douglas, director of the National Literacy Trust, said: "Pupils studying for GCSEs need to be strong enough readers to understand their course textbooks and comprehend exam questions.
"By failing to ensure all young people have the literacy skills they need to access their education, we could be depriving them of the opportunity to succeed both academically and in life.
"Children's reading must be supported throughout their time at school to help them succeed across the curriculum."