Secondary school pupils in Scotland are not reading challenging enough books and potentially harming their ability to properly understand exam papers, according to a literacy study.
Many 16-year-olds sitting National 4 and 5 exams have the reading ability of a 13-year-old or lower, Renaissance UK said.
The literacy and assessment provider is calling for more dedicated time for reading in secondary school after looking at the reading performance of 29,524 pupils across Scotland for the What Kids are Reading Report.
The study found an emphasis on literacy development in primary schools which sees pupils typically read more advanced books for their age but progress largely stops when they reach secondary school with the difficulty of books falling as pupils get older.
The decline exists among both boys and girls and across the UK as a whole but could be reduced by reading for just 15 minutes per day, Renaissance UK said.
Keith Topping, professor of educational and social research at the University of Dundee, analysed the results of the study.
He said: "To avert a further slide in literacy levels in secondary schools, pupils should be encouraged to push themselves to read more difficult books.
"By their teenage years pupils are more likely to take advice from their friends and peers than their teachers and parents about the types of books they should be reading.
"With this in mind, teachers could encourage them to talk more openly about what they are reading and make appropriate suggestions to their classmates."
The Renaissance study found pupils in secondary school often read books that are no harder than those in primary school.
David Walliams' books and Jeff Kinney's Diary of a Wimpy Kid series both dominate the top ten books read by both primary and secondary pupils in Scotland.
The First Minister's Reading Challenge was created by the Scottish Government to promote literacy and "reading cultures" in schools, libraries and communities.
Renaissance UK managing director Dirk Foch said: "The vast majority of primary schools place an emphasis on developing pupils' literacy skills. However, this is rarely continued once pupils go to secondary school.
"This makes maintaining literacy levels among teenagers a persistent challenge for teachers and policy makers.
"The fall in teenagers' reading ages is striking. By the time many come to sit their National 4 and 5 examinations, many will have a reading age of 13 or less, meaning that they could even struggle to comprehend their exam papers.
"This could have a significant impact on their future academic success.
"Evidence shows that pupils make the most progress when they read for just 15 minutes per day, so I would encourage all secondary school teachers to build some dedicated reading time into the timetable to avert a further fall in literacy levels among young people."
A Scottish Government spokesperson said: "Improving education and raising standards for all is this government's number one priority.
"That is why our education reforms have a relentless focus on literacy and we are making a significant investment, through the Attainment Challenge and Pupil Equity Funding, to close the literacy attainment gap."