The independent study, conducted by Professor Keith Topping, University of Dundee, comes at a time when the UK has slipped down to 25th place in the OECD ranking of reading, and statistics from the Department for Education show that more than half of all five-year-old boys in England are failing to make sufficient progress at school.

Professor Keith Topping, Professor of Educational and Social Research, University of Dundee said: "There are some disturbing signs regarding difficulty of books. Although in a small number of years the difficulty of books remained the same as in our study in 2010, in the majority of years the difficulty of books has sharply declined. If we are to address the worrying decline in reading skills identified in the Programme for International Assessment (PISA) world ranking for Britain, we have to ensure our children are reading at or above their reading age."

Commissioned and published annually by Renaissance Learning this is the third report to provide detailed information on the books that children read and their comprehension levels. The report contains data from 150,220 school children and was based on 1,860,440 assessments taken by UK pupils between August 1, 2009 and July 31, 2010. The study found that, in the first four school years, the difficulty of books read was above what would be age-appropriate, but from Year 5 onwards, the difficulty generally declined steadily.

Findings from the report itself have shown, for the second year running, that the gap between gender differences in reading is starting to close, with boys progressively reading as much as girls, although boys are revealed to be reading at a much lower level than girls as they get older.