The accuracy of primary school league tables and data received by secondaries has been thrown into doubt by revelations about material used in the controversial new single-level tests for 11-year-olds.
Results of thousands of pupils from the experimental tests will be included in performance tables for the first time next year.
But The TES has learnt that they will allow pupils to record the achievement of a national curriculum level even if most of their marks were gained at the level below.
The news has prompted a second teaching union to suggest schools pull out of the single-level test (SLT) pilot scheme.
An official report shows that, despite their name, SLTs actually contain questions set at two national curriculum levels.
About 40 per cent of the marks available in the tests, which ministers want to replace Sats, are "available for attainment at the level below that of the test", the Qualifications and Curriculum Authority (QCA) has admitted.
It has also revealed that pass marks for the various tests in maths, reading and writing have been set as low as 29 per cent, raising the possibility of pupils being awarded a national curriculum level despite answering none of the questions relating to it.
The authority claims this is "consistent with the intention underlying single-level tests: to provide a pupil with an opportunity to demonstrate performance at a particular national curriculum level".
But teachers' leaders say it only demonstrates the inherent impossibility of the Government's goal.
The stakes are particularly high because, as The TES revealed earlier this summer, maths SLT results from up to 370 primary and middle schools piloting the tests will be used to compile next year's league tables.
John Bangs, NUT head of education, said: "This will just make the performance tables even dodgier than they already are."
Mary Bousted, Association of Teachers and Lecturers general secretary, said the news would increase existing doubts about the SLTs. "If schools are unhappy about the pilot, they should pull out," she said.
The report relates to the December 2008 tests - the first of four rounds that schools can enter for the 2010 league tables.
It shows that although about 40 per cent of marks related to a lower national curriculum level in each test, the pass marks were set as low as 29 per cent in the reading tests and 48 per cent in the writing.
For maths - the tests that will be used for league tables - pupils only needed to get 52 per cent to pass the crucial level 4 test; they could gain 77 per cent of their marks at level 3 and still achieve level 4.
The QCA was unable to provide pass marks for the June 2009 tests. It said they would change in future rounds, but could not say to what.