More than four out of ten schools trialling a controversial new style of national tests have followed unions' advice and pulled out, it has emerged.
The news came this week as the Government announced it was extending its experiment in single level tests (SLTs) - sometimes also known as "progress tests" - for a further year.
The move could see reading tests, which in trials have had pass marks as low as 29 per cent, used to calculate schools' league table positions next year.
A total of 377 primary and middle schools were originally involved in the SLT pilots. But last summer, when it emerged that results from the experimental tests would be used as accountability measures, two unions advised schools to pull out.
This week the Department for Children, Schools and Families (DCSF) revealed that only 225 schools remained.
Numbers could fall further as none have been confirmed for the 201011 extension year.
The department said the drop-out schools had had concerns about the impact the experimental tests could have on league table positions.
A spokesman said: "We took steps to address their concerns, but ultimately this was a matter for schools to take a view on."
Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said: "This is an incredible drop-out rate. Why are schools leaving if the tests are as worthwhile and valid as the Government says? The low numbers remaining cast doubt on the whole exercise."
SLTs were originally designed in 2007 as an alternative to traditional SATs that would take the pressure off teachers and pupils by allowing them to be entered for a test measuring a particular national curriculum level when they were ready.
Last year it emerged that the DCSF had continued with the trials despite warnings from the Qualifications and Curriculum Authority (QCA) and five independent academics of "substantial and fundamental" problems and that the central aim of the tests was "very probably impossible" to achieve.
In July when The TES revealed results from SLT maths tests would be used to calculate pilot schools' 2010 league table positions, Mick Brookes, general secretary of heads' union the NAHT, said they should pull out.
The ATL followed suit the next month when The TES revealed that the tests would allow pupils to say they had achieved a particular national curriculum level without answering any questions that related to it.
About 40 per cent of the marks in SLTs are "available for attainment at the level below that of the test", the QCA admitted.
For maths tests taken in December 2008 - results of which will be eligible for 2010 league tables - it meant pupils could gain 77 per cent of their marks on level 3 and still gain a crucial level 4.
The anomaly is likely to be magnified for reading and writing SLTs, which could be used for pilot schools' league table positions in 2011.
In the December 2008 test round, the SLT pass mark was just 29 per cent for reading and 40 per cent for writing.
The DCSF spokesman said the pilot was being extended because: "We haven't learned sufficiently about school behaviour, particularly within an accountability context, to enable us to make a decision on the future of SLTs."