There is an "urgent need" to tackle the problem of pupils leaving primary school without the basic skills in reading and writing, say inspectors.
Standards in English are improving, say inspectors
An Ofsted report into English standards says there have been "marked improvements" in England's schools.
But a fifth of 11-year-old pupils, particularly boys, still fail to reach the expected level for English.
Schools Minister Jacqui Smith said the government would be addressing the need to help pupils who were falling behind.
Even though the number not reaching the expected literacy levels has been slowly falling over recent years, the inspectors warn of the impact on secondary schools.
Without being able to read and write adequately, these pupils will struggle with other subjects, and secondary schools will need to develop ways to help these pupils catch up, inspectors say.
The report examines standards in English in schools between 2000 and 2005 - and says that there has been an overall improvement.
But it highlights the pockets of failure below the surface of a broadly-positive picture, in which national strategies have contributed to "significant improvements".
"English is one of the best taught subjects in primary and secondary schools," the report says.
But despite this, in primary schools, around 30% of lessons are "no better than satisfactory".
"There has been a marked improvement in the reading standards," the report concludes.
Nonetheless, reading for pleasure is in decline.
"Too few schools have given sufficient time and thought to how to promote pupils' independent reading and there is evidence that many pupils are reading less widely for pleasure than previously," the report said.
"Many teachers struggle to keep up-to-date with good quality texts for their pupils to read."
Inspectors report on the continuing literacy gap between girls and boys, which they say is particularly pronounced in writing - where they say boys should be given extra help.
"Evidence from the most effective schools suggests that more can be done in many schools to improve the standards of boys' writing."
There are also concerns in the report about black pupils who make only "limited progress".
The teaching of phonics - letter sounds - showed "significant variation".
So Ofsted was offering support to the government-commissioned Rose Review of phonics teaching, taking place this autumn, to guide teachers in producing the highest reading standards.
In terms of test results, the period covered by the report has been one of slowing progress - with results getting better but falling behind government targets.
The schools minister said the government would be setting out in a forthcoming White Paper "further support to help schools with catch-up programmes to ensure that all pupils make progress".
Shadow Schools Minister Nick Gibb said: "Despite the all the rhetoric and statistics this report reveals that serious problems remain with the way primary school children are taught to read."