Ministers came under renewed pressure to scrap Sats tonight as results showed reading standards among 11-year-olds slipped for a second consecutive year.
More than a third of pupils also left primary school without a proper grasp of the basics in reading, writing and maths, National Curriculum test results revealed.
But youngsters are getting better overall at English and maths.
The Government was forced to disregard results in more than 20 local authorities, which were rendered "statistically unrepresentative" by a boycott which saw tens of thousands of children not sitting the tests.
The results showed only 65% reached the standard expected for their age in reading, writing and mathematics combined.
The results showed:
:: 84% of 11-year-olds achieved the required level in reading, down from 86% last year and 87% in 2008.
:: 71% were up to standard in writing, up from 68% last year.
:: 81% hit targets in English, up from 80% last year;
:: 80% reached this level in maths, up from 79% last year.
Christine Blower, of the National Union of Teachers, dismissed the test results as an "irrelevance" and praised the quarter of English schools which pulled out.
She added: "As much as £20 million in public money would be saved by a move to sample testing, which would be significantly more valid and not create a distorting effect on the curriculum.
"Parents are more interested in their children's happiness, security and progress across the curriculum than they are in the results of the deeply flawed Sats."
Ms Blower said improvements in scores in English and maths were not down to changes in standards.
She said: "The marginal shifts in percentages paint a picture of the vagaries of test questions rather than a change in standards.
"Teachers know from their own assessment that a minority of children need intensive support such as one-to-one tuition; they don't need irrelevant tests to tell them that.
"Indeed, research shows that Key Stage 2 test results have a significant margin of error."
Schools Minister Nick Gibb defended the role played by the tests but admitted support for teachers was needed.
He said: "Despite pupils' and teachers' hard work, one in five pupils are still not reaching the expected level in either English or maths and over a third are not achieving this level in reading, writing and maths combined.
"We need to ensure Government gives teachers the support they need to get the basics right.
"Getting the fundamentals right - being able to read and write and having a solid foundation in maths - is crucial to a child's success in secondary education and throughout their adult life.
"This is why the coalition Government is committed to promoting the use of systematic synthetic phonics in primary schools and to ensuring that pupils are fluent in arithmetic and basic maths by the time they move to secondary school. We will provide the help teachers need to do their job even better."
Unlike last year, teacher assessments on pupil performance over the year were published alongside the test results for around 600,000 youngsters.
The teachers said 81% of pupils were achieving the required level in English and maths, while 85% were reaching the standard for science.
At 14, 79% were hitting targets for English, while 80% were reaching the required level in science and English, the teachers' assessments showed.
The Department for Education said 4,005 of the 15,515 schools which were expected to administer the tests did not do so.
Shadow schools minister Vernon Coaker said English and Maths results were "encouraging" and "the culmination of a transformation in school standards thanks to Labour's investment and reforms".
"But there is obviously more to do, particularly in reading where the results are disappointing," he added.
"These results show why the coalition's cuts to the budgets of successful catch-up programmes like Every Child a Reader, which we were rolling out across the country, are so short-sighted and disastrous for educational opportunity."
Chris Keates, of teachers' union NASUWT, said "all involved should be congratulated and their work celebrated".
But he added: "The NASUWT remains of the view that publishing teacher assessment data alongside externally marked test results will cause confusion.
"This is a fundamentally flawed idea which was proposed by certain headteachers in an attempt to shift accountability from the school to individual teachers.
"Publishing this information leaves individual teachers exposed to criticism of their professional judgment and opens up the potential for their professional reputation to be unfairly impugned.
"The publication of teacher assessments should be halted and the NASUWT will continue to campaign for this."
Dr Mary Bousted, general secretary of the Association of Teachers and Lecturers, said: "Today's figures confirm what we have been arguing, that teacher assessments can be as accurate as test results and teachers' judgment both can and should be trusted.
"The real problem with Sats remains their high stakes nature. The elephant in the room is that any method of assessment which is used to judge teachers, schools and pupils like this will have the same outcome - children being taught to the test and stressed children and teachers."