Almost a third of boys never pick up a pen and write for fun outside of the classroom, according to research.
But nearly a third of girls do enjoy spending time writing.
The National Literacy Trust, which published the findings, said that the trend against boys writing could have a negative impact on their schoolwork.
The charity's fourth annual literacy survey, which questioned nearly 30,000 eight to 16-year-olds, found that young people who write for fun on a daily basis are four times more likely to be writing above the level expected for their age group than youngsters who never put pen to paper outside of school.
Boys were twice as likely as girls to say they don't enjoy the activity - 19.2% compared to 8.2%.
And 31% of boys said that they rarely write for fun outside of lessons, while 32% of girls write daily.
Almost one in five boys (19%) admit that they would be embarrassed if their friends saw them writing, and were also less likely than girls to agree that writing is "cool".
Jonathan Douglas, director of the National Literacy Trust, said: "Our research shows that we must focus on increasing boys' frequency and enjoyment of writing if we are to support them to succeed at school and throughout their future lives.
"It's down to teachers as well as parents to nurture a love of writing in boys and help to develop positive attitudes towards it early on in their education."