The Government has been accused of ploughing ahead with politically-motivated league tables even though its curriculum quango is still experimenting with this week's controversial tests.
Headteachers have seized on the news as further evidence that the tests are far from "bedded down" as ministers claim.
The School Curriculum and Assessment Authority has confirmed that uncertainty about comparisons with last year means it must wait until it has seen 10, 000 completed test papers before deciding how well schools have performed.
The admission is a major boost to the National Association of Headteachers whose campaign against primary league tables hinges on claims that the tests for 11-year-olds are still experimental. This week the NAHT urged governors in 14,000 primary schools to ignore the law and obstruct the league tables.
SCAA's decision not to award final grades until it has seen the raw results has alarmed many of the 12,000 external examiners.
It has led to allegations that the final printed grades will be open to political manipulation - in an election year - because the numbers of pupils reaching good grades can be determined in retrospect.
Gillian Shephard, the Education and Employment Secretary, announced in February, after a tabloid newspaper campaign, that there will be primary performance tables of results in English, maths and science. Yet only 11 days earlier she had said there would be no league tables because the tests had not yet "bedded down".
The Welsh Secretary has taken a contrary view and will not be publishing 1996 results for Welsh schools.
This year's tests have been subject to substantial changes and already some primary heads are complaining that the science tests are unreasonably hard. Last year's were judged comparatively easy.
David Hart, general secretary of the NAHT said there is particular concern about attainment level 4, the "average" grade on which schools will be judged. He pointed to last year's 10 per cent difference between the externally-marked results and teacher's own assessment in maths. There was also a 26 per cent disparity between maths and science at level 4. "The implication is that until they have sampled 10,000 scripts, they're not going to fix the crucial level 4 bench-mark. This is worrying. It shows they have still not got it right. It is also bound to raise suspicion that SCAA, in concert with the Government, could be moving the goal posts.
David Hawker, assistant chief executive of the SCAA, said that the additional work on the tests amounts to fine-tuning. "We're clear that the standards we set have to remain fixed from year to year. But to do so we have to calibrate our tests - using pre-test data - so that they compare with the previous tests.
"We're doing a double check to make sure that we have not misinterpreted the data. There is a misconception that we're fixing the results. Actually it's the opposite. We're simply recognising the need to do an open and professional job."
The NAHT this week called on governors to "stand up and be counted" in a high-risk strategy to sabotage the league tables.
It is asking governors to break the law and refuse to return test results to the Government's official marking agencies.
"The decision of the Government to reverse its policy is in my view nothing more or less than a blatant piece of political opportunism," said Mr Hart.
The action places Mrs Shephard in a politically difficult position. Anxious not to offend governing bodies or her backbenchers, the DFEE this week said only "we are expecting governors to cooperate".
The NAHT's call for governor action has been condemned by the National Governors Council and the National Association for Governors and Managers.