Teachers have revealed that pupils did well in this summer's key stage 2 English tests, even though they had not mastered the basics of reading and writing.
Primary staff have used the TES online staffroom forum to describe how their schools' English results were way above their own assessments of their pupils' achievements.
Some teachers said they pitied staff in secondary schools for having to show progress with these pupils when they transfer. Others said there was a political agenda to make the results look good.
One contributor said: "I nearly fell over when I heard (that we had) 96 per cent level 4 plus and 69 per cent level 5.
"This makes a mockery of my very accurate teacher assessment. There are children who write largely in simple sentences, with scarcely an adjective or comma in sight and they have been given low level 5s. These grades are not accurate."
Another said: "We had 34 level 5 writers (in the test results) when we had only six on our teacher assessment, and 11 predicted to get a level 3 when only one did."
Jim Knight, the schools minister, has admitted that national test results were not an accurate measure of all pupils' true abilities.
In a letter to the Association of School and College Leaders, he said: "I accept that no test or examination is 100 per cent reliable. It is well-known that some pupils will perform better or worse than their 'true'
level of attainment on any particular day."
Mr Knight's admission was seized upon by opponents of testing who said that teacher assessment gave a better guide to achievement.
John Bangs, National Union of Teachers' head of education, said he had never heard a minister admit before that tests were in any way unreliable.
"According to the Government, the tests are supposed to provide a constructive and accurate picture of a child's achievement," he said.
"Clearly, it does not think that is the case for a large number of pupils.
The results should not be used as benchmarks for performance tables."
John Dunford, ASCL general secretary, said: "Test results have assumed a level of importance which is way beyond their statistical accuracy."
As The TES revealed last week, the KS2 English threshold for level 4 rose from 42 to 43 per cent this year. Pupils only needed 36 per cent to get level 4 in reading. A score of 70 per cent was needed to achieve a level 5 in English, the highest for four years.
Many schools were happy with their KS2 results in core subjects, revealing scores of 100 per cent level 4s. Pupils' test performance is central to target-setting. Small primary schools have complained that they can suffer if a small number of pupils perform badly on the day. Results are also used to target extra provision according to pupil "ability" levels.
The Government is to contact secondaries within two weeks to tell them which pupils were in the top 5 per cent in KS2 tests, creating a national register of high achievers. Schools will be expected to ensure that these pupils fulfil their potential, although they will be free to add the names of those they believe underperformed.
A Department for Education and Skills spokeswoman said the possibility of pupil under- and over-performance was well-known.
She said teachers could use other information such as script analysis, a pupil's numerical score, and teacher assessment to "inform (the pupil's) future teaching and learning", and that the popularity of optional tests showed that teachers valued the information provided.