Lightweight fiction and magazines could be damaging children's ability to write good English, a government report says.
It added that teachers felt the "informal style of pupils' personal reading diets" pervaded some pupils' writing - making it too colloquial.
And there were claims that the language of texting and e-mailing was being used when a more formal style was required.
The report looked at how best to boost the performance of children who were falling behind in the core subjects.
It is part of a Making Good Progress resource pack being made available soon to schools, a draft of which was released to journalists.
The latest results show that four out of 10 14-year-old boys failed to achieve the expected level in last year's national English, maths and science tests.
A third of 14-year-old girls also failed to make the grade.
The Department for Children, Families and Schools report offers voluntary guidance for schools on how best they can help pupils who are at risk of not meeting these standards.
It looks at the experiences of 285 pupils in 43 secondary schools across England whom teachers considered to be "slow moving".
The report said these pupils read lightweight fiction and magazines at home for pleasure, which some teachers regarded as "comfort reading".
Teachers said they were concerned about the impact on pupils' writing, which tended to be "inappropriately colloquial" when the task required a little more formal style.
This could be addressed by making pupils realise that writing is not simply "talk written down", the report added.
Teachers should also give pupils the chance to use spoken and written language in formal as well as informal situations, the report added.
Poor literacy skills had a negative influence on other subjects too, the report suggested.
It was often seen as a key obstacle to progress in mathematics and science.
Those struggling in maths also lacked confidence and the vocabulary to talk about maths.
The report also said these pupils struggled with "mental calculation" and were put off numbers larger than 100 because they were too big.
But equally they needed more experience of using calculators and had difficulty in interpreting the display, it added.
Those struggling in science also tended to have problems with reading and writing.
Many of them simply could not remember what they had studied from one week to the next.
And pupils said they spent most of their science lessons listening to teachers and did little group activity.