National test results for 11-year-olds in English have remained largely static for the fifth consecutive year.
Eighty per cent of pupils reached the expected level 4 in English - the first dip recorded since the current tests began in 1995. But since 2005 the English scores have ranged between 79 per cent and 81 per cent.
In maths and science, the numbers are exactly the same as in 2008 - 79 per cent and 88 per cent, respectively.
Diana Johnson, schools minister, said the figures mean neither government nor schools should be complacent.
She insisted the department's portfolio of policies - including one-to-one tuition for struggling pupils, more freedom for schools after the demise of the National Strategies and the new primary curriculum - will drive results up.
There were also disappointing results for the Government this year. For example, ministers had set a target for 78 per cent of pupils to reach level 4 in both English and maths by 2011. This year, just 72 per cent did so.
Unions, however, insist these headline results give little useful information because the tests are outdated.
The NUT and National Association of Head Teachers are both planning to boycott the tests next year if there is no movement on changing the system, although the NASUWT has said the problem is not the tests but the league tables.
Teachers and heads will be particularly wary of the results this year after the fiasco in 2008, when contractor ETS delivered the results late. The US firm lost the pound;156 million five-year contract, and former contract holder Edexcel was brought in to run the tests for this year only.
However, Ian Foster, NAHT assistant secretary, said: "We have been contacted by more than 100 schools where members are concerned about the inconsistent marking in English."
When English is broken down into reading and writing, the figures show 56 per cent of boys leave primary school with level 4 in reading, writing and maths, compared with 67 per cent of girls.
The proportion of children achieving the higher level 5 in English has been more variable. The figure has ranged from 27 per cent in 2005 when the tests included writing a play about a boy asking his father if he could stay up late to watch television to 34 per cent when the task was to design a leaflet about recycling.
Christine Blower, NUT general secretary, said: "The truth is that anyone trying to get a true picture of primary school achievement from these results will have a very hard job.
"The marginal shifts in percentages paint a picture of the vagaries of test questions rather than any change in standards."
Maths scores have plateaued at 79 per cent, after rising from 75 per cent in 2005.
This year, 35 per cent of pupils reached the higher level 5 in maths, the highest score recorded and a jump of four percentage points on last year.
In this, the final year of the science test, 88 per cent of pupils reached level 4.
This year's results cover 577,000 pupils and are provisional. They may change by a single percentage point up or down depending on the result of appeals.