Test results for 11-year-olds published yesterday showed a drop in reading standards for the second year running.
The number of children achieving the required standard fell from 86 per cent to 84 per cent this year, following a 1 percentage point drop in 2009.
Education heads believe that the drive to boost writing standards, which still lag far behind reading, may have caused teachers to focus less on reading. The Schools minister, Nick Gibb, described the drop in standards as "disappointing".
Overall performance in maths and English both increased by 1 percentage point, but officials said the figures should be treated with caution because some schools chose not to make pupils sit the tests this year.
According to the Department for Education, just over 4,000 schools, or 25 per cent, refused to administer the tests as a result of a boycott by both the National Union of Teachers (NUT) and the National Association of Head Teachers (NAHT). They complained that too much teaching to the tests was ruining children's education as schools strived to do well in league tables.
A breakdown of the figures showed that 81 per cent of pupils reached the required standard in English and 80 per cent in maths, compared with 80 per cent and 79 per cent last year. But within English, there was a fall in reading of 2 percentage points to 84 per cent. Writing, which has been the subject of targeted aid to improve performance, rose by 3 percentage points to 71 per cent.
This year's results compared teachers' assessments of their pupils with tests for the first time. However, there was little difference in the two figures, with 81 per cent reaching the required standard in both subjects as a result of teacher assessments.
Mr Gibb congratulated the children on their hard work. But he noted that more than a third of all pupils (35 per cent) had failed to reach the required standard in all three tests: maths, reading and writing. "We need to ensure Government gives teachers the support they need to get the basics right," he said.
Christine Blower, general secretary of the NUT, said: "The actions of over 4,000 schools [who boycotted the tests] will be sufficient to render this year's league tables an irrelevance."
Russell Hobby, general secretary designate of the NAHT, said: "Over a quarter of schools boycotted the tests this year because of a firm conviction that they are a deeply flawed method of assessment.
"They don't show how well children are learning, they don't show how good a school is, they divert time and attention from learning to testing and they punish the leaders and teachers who work in our most challenging schools and disadvantaged communities."
Education Secretary Michael Gove has promised a review of the tests. However, he insists they will go ahead next year, even if there is another attempt to boycott them.