Children who write for pleasure achieve significantly better results in the subject in the classroom, National Literacy Trust research suggests.
Those who like writing outside class are seven times more likely to write above the expected level for their age.
While the proportion of children writing for fun has risen, the trust warns many are still not keen on it.
It says more attention must be focused on writing for fun, as has already been done on reading for pleasure.
The study, published to mark the first National Writing Day organised by the charity First Story, questioned 39,411 eight to 18-year-olds across the UK.
The survey found almost a quarter (23%) of those who said they enjoyed writing were above their expected level, compared with 3% of those who said they did not like the activity.
While the proportion of young people who do enjoy writing has risen – from 45% in 2015 to 51% in 2016 – the NLT study suggests about half (49%) do not enjoy writing outside of the classroom.
Focus on writing
NLT director Jonathan Douglas said: "Our research consistently finds that children who enjoy writing do much better at school, but it also shows that far too many pupils still don't enjoy writing and this could be holding them back from reaching their full potential.
"For the past 20 years, we've seen a real focus on reading for enjoyment initiatives across the UK, which have reaped fantastic benefits for children.
"It's now time to give writing for enjoyment the focus it deserves."
The results suggest girls are more likely than boys to enjoy writing, and this gap widens the older children get.
The pupils aged eight to 11 were more likely to enjoy the activity very much compared with those aged 14-16, suggesting enjoyment of writing decreases as children grow up.
The pupils from white backgrounds were less likely to enjoy writing than were those from mixed, Asian and black backgrounds, the study found.
While more children from less privileged backgrounds – those eligible for free school meals (FSM) – enjoyed writing very much or quite a lot compared with their richer peers.
Overall, 54% of the FSM pupils said they enjoyed writing either very much or quite a lot, compared with 50% of the non-FSM pupils.
The NLT said there were a number of obstacles that stopped youngsters putting their thoughts down on paper.
About 44% of those who said they enjoyed the activity and 56% of those who did not like it struggled with deciding what to write.
And 30% of those who liked writing and 35% of those who did not said that they had trouble with spelling and grammar.
Comedian and author Charlie Higson, who is supporting National Writing Day, said: "There's been a big focus on reading as an aid to literacy, and there are no end of events, festivals and initiatives promoting it.
"It's fantastic to have something that actively encourages writing."
Earlier this month, the NLT published research suggesting just a third of teenage boys in the UK enjoyed reading.
The trust found a significant drop in boys' reading enjoyment between the ages of eight and 16.