A third of 11-year-olds in England are getting higher scores in English, mathematics and science national curriculum tests than teachers believe they deserve.

The findings from the think-tank Civitas will undermine further the legitimacy of the national Key Stage 2 results, published today. They have already been hit by administrative errors in the marking and provision of results; last night 17,400 Key Stage 2 test results were still missing.

Today’s results - the first national test results for children born in the academic year that Labour came to power in 1997 - are expected to show an increase in the proportion of children achieving Level 4, the standard expected of their age, but a drop in the number achieving the above-average Level 5 grades.

The results come amid signs that the Government is preparing to dump ETS, the private contractor that took over the marking of the national curriculum tests this year. A source said that the Government was “in serious talks about serious talks” to find a way to end the five-year contract early.

The research from Civitas suggests, however, that it will take more than a change of contractors to iron out problems with the Key Stage test results.

According to its survey, at 107 secondary schools, 79 per cent of Year 7 teachers found that up to a third of their year-groups were less able than their Key Stage 2 results indicated.

Most blamed the practice of teaching to the test or “coaching” pupils in Year 6, which gave pupils an unnatural boost on test day, according to Anastasia de Waal, the report’s author.

“Primary school teachers are finding themselves compelled to teach to the test both through official guidance and through pressure to do what they can in the short-term to gain higher SATs scores,” Ms de Waal said.

Testing was supposed to act as a snapshot of learning – but today that snapshot is often “the sum of pupils’ learning”, she added.

Because secondary schools could not rely on the Key Stage 2 results to provide an accurate indicator of their Year 7 pupils’ ability, the majority routinely ran their own tests in English, maths and science, the report found.

The solution is not to scrap primary testing, Ms de Waal says, but rather to introduce a system in which schools are not forewarned on either details of the test content, or their timing.

Today’s Key Stage 2 results are expected to show that about a quarter of pupils aged 11 failed to reach the level expected of their age in maths. About one in five has failed to do so in English.

Michael Gove, the Shadow Children’s Secretary, said it was a tragedy that the gap in achievement between children eligible for free school meals and those from the middle classes had steadily grown under Labour. “Pupils who have taken their SATs tests this year were born in the year that Labour came to power. Yet hundreds of thousands will not have reached the basic level of literacy and numeracy, and the gap between the most disadvantaged pupils and the rest continues to grow,” Mr Gove said.

Ministers are expected today to emphasise that the Key Stage 2 results are provisional. They will also point to the continued insistence of the exams regulator Ofqual that there is no evidence of widespread problems with the quality of the marks at Key Stage 2.

But Mick Brookes, the general secretary of the National Association of Head Teachers, said that the Government’s decision to press ahead with the publication of results “beggared belief”. “We have received 400 e-mails from heads highlighting their concerns about the results,” he said.

Teachers are also concerned about the possible impact of the ending this year of the process of “borderlining”, whereby any script scoring just below the national curriculum level would be remarked. Government experts had suggested that the decision could lead to a drop in overall results of two percentage points. But there is now considerable uncertainty over the effects of the change.

Education, education, education

1.2 million 11 and 14-year-olds taking the tests in England in 2008

9.5 million Key Stage 2 and 3 papers in English, maths and science were sat this year

2.85 million children have not achieved the expected Level 4 in reading, writing and maths at Key Stage 2 since 1998

7 days the “official” delay in releasing Key Stage 2 results to schools this year

7,800 Key Stage 2 English test results are still missing

£156 million paid to the US-owned contractor ETS to mark the papers for five years

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