Thousands of 10 and 11 year olds are expecting their results from May's exams next week.
But questions have been raised about the way they are marked after a report from Ofqual found that a significant number of papers were wrongly graded in previous years.
Analysis of the 2008 English exams revealed that up to 17 per cent of pupils could have been "misclassified" when their papers were marked.
The figure was seized on by teaching leaders, who said the exams ought to be scrapped because they are not a reliable form of assessment for children.
Last year the exams regulator admitted that up to half of pupils are given the wrong marks in the key stage two and three tests every year, with English papers the worst affected due to subjective marking.
In the latest study, a group of 1,387 pupils who took the English reading exam in 2007 were asked to also sit an advance copy of the paper which would be set the following year.
Analysis of the results showed that only 83 per cent of the pupils were classified correctly for the 2008 paper, meaning almost one in five was given a grade that did not reflect their ability.
Similar tests for science papers carried out between 2004 and 2008 showed that between 20 and 30 per cent of pupils each year were given different grades for their real exam as for the following year's paper.
The survey prompted fears that similar inaccuracies will be allowed to take place in the marking of this year's exams.
Martin Johnson, deputy general secretary of the Association of Teachers and Lecturers (ATL), said: "Ofqual’s report backs up what ATL has been saying for a long time – that a substantial percentage of key stage 2 test results are unreliable.
"Relying on test scores is highly dangerous when they are flawed and the more uses to which they are put the more flawed the conclusions. The government would be incredibly foolish to continue to keep its head in the sand and ignore this.
"A sensible government would accept all the irrefutable evidence and consign SATs to the dust bin of history.”
The tests have been widely criticised for being too restrictive and for putting too much pressure on children at a crucial stage of their education.
Up to a quarter of schools boycotted the exams last year, claiming they narrow the curriculum, hamper children's learning and put head teachers' jobs at threat due to their impact on league tables.
Earlier this month two teaching unions called for the tests to be scrapped in favour of a system where teachers would assess their pupils' progress, but Michael Gove, the Education Secretary, has pledged to press ahead with the exams in English and Maths in 2011.
--By Nick Collins,26 Jul 2010