Schools are "queuing up" to complain about the quality of marking in this year's key stage 2 Sats, which is making a "mockery" of the efforts of teachers and pupils, according to a headteachers' union.
A catalogue of errors has been reported, with pupils of all abilities awarded the wrong grades, according to the NAHT.
Union general secretary Russell Hobby described the situation as this year's "second exam fiasco", after a series of mistakes in GCSE and A- level papers.
"Inconsistencies abound this year, along with stark examples of low quality control," said Mr Hobby.
"As a nation, we pour massive sums of money into setting and marking Sats - money which could be far better spent on teaching children."
In a poll of heads in Lancashire carried out by the union, 74 of 77 members who responded reported "serious inconsistencies" in marking, with the writing test causing particular alarm.
The growing concerns about marking quality follow the publication of the Bew review of KS2 Sats, which recommended that the writing test is scrapped in its current form and replaced by teacher assessment.
The mistakes in grading the writing test are "another nail in the coffin for testing as a gold standard of assessment", Mr Hobby said. "Certainly the case for the end of the writing test is now cut and dried."
The mistakes reported to the NAHT come as the head of Cranmere Primary School in Esher, Surrey, wrote to education secretary Michael Gove to complain about what she called the worst examples of incompetent marking in 18 years.
Gillian Freeman told Mr Gove "the system is a disgrace and an insult to the professionalism of my teachers" and asked him to accept the recommendations of the Bew review.
Mrs Freeman said around 10 of the 26 papers completed by her pupils will be sent back for re-marking this year.
She included two papers from the 45-minute writing test, which asked children to write a newspaper report.
One was graded in the lowest band for composition and effect, despite being a lengthy account that included the views of different people.
The other paper, which lacked the same detail, was graded at the top of the next band up.
"We have to report this mark back to the child and we won't get the results of the appeal until September," Mrs Freeman said. "That child will go all through the summer holidays knowing they are assessed at level 3. It is wrong.
"Quite a lot of parents are swayed by these statistics, and if they are based on nonsense, as has happened this year, it just makes you despair."
A Department for Education spokesman said: "Markers, who are practising or retired teachers, receive thorough training and undergo a number of quality checks through the marking process."
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Original headline: Sats `fiasco' in the making as schools `queue up' to complain