A quarter of eight- to 18-year-olds rarely or never write something that is not for school, a report says.
Just one child in five told a survey for the National Literacy Trust they wrote daily outside of school in 2015, compared with 27.2% in 2014.
And that writing was dominated by social network posts and text messages, the survey of 32,500 pupils suggests.
The trust warned the development could have a negative impact on pupils’ results.
Those who write outside school daily were five times more likely to have writing skills above their age group, it added.
And those who enjoyed writing were seven times more likely to have advanced writing skills.
The report said: “Since we started measuring writing frequency as part of the annual literacy survey, in 2010, the percentage of children and young people who write daily outside of class has remained relatively stable, with just over a quarter of people saying that they write something outside of class daily.
“However, in 2015, there was a significant change in the frequency with which children and young people write outside class, with fewer children and young people in 2015 writing as frequently as their peers did in 2014.”
The survey did not just focus on children putting pen to paper, but instead included writing on technology-based formats.
Text messages were the most common form of writing children indulged in, followed by emails, social network site messages and instant messages.
Children most commonly put pen to paper in the form of notes, with a third of children writing them at least once a month.
Letters were the next most common form of writing, with a quarter of children writing letters at least once a month.
Jonathan Douglas, director of the National Literacy Trust, said: “Without solid writing skills, young people will have fewer opportunities open to them, ultimately impacting on their social mobility and indeed on the UK economy.
“If we don’t act now, the futures of children who cannot write well when they leave school could be cut short before they’ve even started.”
The charity is calling for a renewed focus on writing for enjoyment.