Scottish pupils are not reading challenging enough books at secondary-school level, according to a new study.
The reading age of 16-year-olds typically fell to at least three years behind, the study of 26,000 Scottish students found.
Dundee University's Prof Keith Topping carried out the study for online assessment provider Renaissance UK.
According to the company's What Kids Are Reading report, the trend was reflected across the UK.
Prof Topping found that primary-school students typically pushed themselves to read more complex books than older pupils.
He said: "The brain is a muscle that literacy skills help train.
"As it gets more toned, like all muscles, it needs more exercise.
"Currently, primary schools are exercising it more vigorously by reading more challenging books - we now need to replicate this in secondary schools."
Prof Topping said more discussions between young people about books they were reading should be encouraged.
He said: "I would also encourage all secondary teachers, not just English teachers, to look closely at their pupils' literacy levels and remember - even the brightest students need to be stretched."
The report studied the reading habits of 848,219 young people across almost 4,000 schools in the UK.
Renaissance UK managing director Dirk Foch said: "Most primary schools place a large emphasis on developing literacy skills.
"However, this is rarely transferred onto secondary school and, as a result, literacy standards at secondary level are a persistent challenge."
A Scottish government spokesman said: "We welcome this report's finding that primary schools are providing challenging books for children to read.
"It is essential young people continue to develop advanced reading skills in secondary, which is why Education Scotland published literacy and English benchmarks last year, making clear what pupils need to know and do at each stage."