Results from experimental tests being taken by thousands of pupils will count in next year's primary school league tables, despite experts' warning of "substantial and fundamental" problems with the new exams.
A heads' leader is advising his members to pull out of the single level test (SLT) pilot scheme after learning of the Government's plan to use the results as a high-stakes performance measure.
The tests are being piloted by 370 primary and middle schools, but they have not previously counted towards the schools' official results and were trialled alongside traditional Sats.
Serious doubts remain over whether the SLTs, designed to measure whether pupils have reached a particular national curriculum level, have been sufficiently evaluated to be ready for use as results that, if bad, could lead to schools closing and teachers losing their jobs.
Questions will also be raised over the fairness of comparing those schools' results in league tables against those offering traditional Sats.
The SLT pilot schools will have had four opportunities to obtain good results from next year's Year 6 pupils in the tests - in December 08, June 09, December 09 and June 10 - while the Sats pupils will have a single attempt.
The Government wants maths SLTs to be used to calculate the pilot schools' league table positions from 2010, but has not said when English results will follow.
It is proceeding with the plan even though there has been no evaluation of the June 09 round of SLTs, which could count towards some official results.
Four trial rounds of SLTs have been completed since December 2007 and the format of tests used has been changed on each occasion.
Last month it emerged that a report of the first trials conducted by the Qualifications and Curriculum Authority and five independent academics warned of "substantial and fundamental" problems.
This week the Department for Children, Schools and Families said it was clear that problems found on key stage 3 SLTs "were not found at KS2".
But a report from the same authors on the second round of SLTs, written last autumn after the decision to axe KS3 SLTs had been taken, warned that the central aim of tests - to assess pupils regardless of age, across a broad curriculum - was "very probably impossible" to achieve.
The QCA said it will "shortly" publish an overarching technical report covering the first three rounds of SLTs. But this had not been completed or passed to the DCSF in May when the department decided SLTs should be used in next year's league tables.
The experiment could have big consequences for pupils who in conventional Sats can achieve a range of national curriculum levels by sitting a single test.
If they fail to pass an SLT they will end up with nothing. So schools could be tempted to play it safe by entering potential level 5 pupils for an SLT measuring level 4 - the key level for primary league tables.
The 370 schools will have until September to confirm whether they want to take part in the extended pilot.
Mick Brookes, National Association of Head Teachers general secretary, said: "If they (SLTs) are going to become high-stakes accountability measures, my advice to pilot schools would be to pull out."
Single level tests - the facts
- Single level tests were first proposed by the Government as alternatives to conventional Sats in January 2007.
- The idea was that they could 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.
- Critics warned from the start that they would increase pressure by leading to more tests.
- Problems and "erratic" results emerged when attempts were made to produce tests appropriate for primary and secondary pupils.