In a verdict which contrasts with what teachers have been told about the key stage 3 test, the Qualifications and Curriculum Authority told the Government in November it had "yet to be stabilised".
In a letter when advising ministers that it should be voluntary, Sir Anthony Greener, its chairman, listed four "challenges" facing the test.
These were that the test development process had "yet to be stabilised"; that work was still needed to resolve its validity and reliability; that schools faced a challenge ensuring they had the hardware to run it; and finally, that many initiatives, all of which would burden schools, were planned for 2008, when the test was due to become compulsory. This has been dubbed "meltdown year", when functional skills tests and specialised diplomas are introduced alongside A-level, GCSE and key stage 3 reforms.
In response, Alan Johnson, the Education Secretary, wrote in December: "I agree that the challenges you identify call for changes to the approach we had set out (in 2005)."
Making the test voluntary would also give teachers a chance to test pupils whenever they were ready, he said, supporting the Government's personalised learning drive.
Last month, announcing the test was being made voluntary, Jim Knight, the schools minister, did not mention any problems.
Teachers have been advised by the National Assessment Agency to run the test as a voluntary assessment this spring. The TES understands that the difficulties included problems judging brighter pupils' abilities.
Many teachers have expressed disgust on The TES online staffroom that the advice is being given without the Government having been more open about the problems, although some view the test as a good assessment.
* The letters between Sir Anthony and Mr Johnson can be seen at www.qca.org.uk12039.html