First the good news: boys are reading as much as girls. Now the bad: the books they choose are far less challenging and easier to comprehend than those selected by girls, and this worsens as they grow older.

The findings of a major study of 100,000 British children's reading habits coincide with national curriculum test results which show that - at all ages - girls score more highly on reading tests.

"Boys are clearly reading nearly as much as girls, a finding that may surprise some onlookers," said Professor Keith Topping, of the University of Dundee's school of education, who headed the study.

"But boys are tending to read easier books than girls.

"The general picture was of girls reading books of a consistently more difficult level than boys in the same year."

The gap in the standard of their reading habits becomes most marked between the ages of 13-16, the report says.

The favourite book for girls in this age group is Twilight, by Stephanie Meyer, the first in the vampire romance series that has sold 85 million copies.

This was ranked far more difficult to read than the boys' favourite, The Dark Never Hides, from British novelist Peter Lancett's Dark Man series, illustrated fantasy novels aimed at reluctant teens and young adults struggling to read.

The study notes that both sexes tend to choose books that are easier to read once they reach the age of 11 and transfer to secondary school.

Compared with a similar study two years ago, the Harry Potter author J. K. Rowling has tumbled down the top 10 most popular children's authors, from second to ninth place.

Boys, in particular, chose not to read her books, which are considered more challenging than many other children's titles.

"Perhaps the lapse in popularity of the Harry Potter books ... has left boys with few high-difficulty books they have the urge to attack," Professor Topping said.

One author to shoot into the top 10 for the first time - at No 2 - is Roderick Hunt, whose 300 The Magic Key books, following the lives of three children and their dog, Floppy, are used in 80 per cent of British schools to teach people how to read. Roald Dahl still tops the chart.