One in four seven-year-olds will be able to opt out of the 2004 tests in May as part of a pilot scheme - but only if schools agree to use last year's papers.
While other schools continue with the compulsory reading, writing and maths tests, schools in the pilot will be able to use either of the two tests at any time in the spring or summer terms.
Ministers announced the pilot scheme in May to counter growing anger about young children being put under test pressure.
The National Union of Teachers said that the pilot is a halfway measure which could strengthen calls for a test boycott. Its executive is due to decide next month whether to ballot for a national boycott of tests following a members' survey which suggests widespread opposition to them.
A similar threat was made by the National Association of Head Teachers, but it backed down after the Government announced its primary strategy which included a pledge to pilot changes to the way seven-year-olds are assessed.
John Bangs, NUT head of education, said that the insistence on using specific tests for assessments undermined teachers' judgments. He said: "If the Government has conceded the point that teacher assessments should stand, why not get rid of the tests?"
David Hart, NAHT general secretary, said: "The pilot will produce such a minimal reduction in the volume of testing that it's highly unlikely to change the attitude of the vast majority of key stage 1 heads who feel very strongly that tests and tasks have no role to play."
Pilot schools will trial the use of KS1 tasks and tests to underpin teachers' judgments.
The pilot will run in 38 authorities. Schools taking part can choose to use either the 2003 papers or 2004 papers - or elements of both - to assess pupils.
Teachers in trial schools also have four options:
* to give children either the level 2 reading task or test, rather than both.
* to allow children who have done well in the level 1 reading task to skip level 2.
* to allow pupils who have done well in in both the level 2 task and test to skip the level 3 reading test .
* a child who fails the level 3 test will not have to take the level 2 test in maths or reading.
There will be no difference in how writing is assessed.
Teacher assessment results will be reported from each trial school for each child. Moderation will continue to be carried out by local education authorities.
The Qualifications and Curriculum Authority, which oversees the tests, has said that it is likely different styles of moderation will be used and evaluated.
The evaluation will look at the impact any change in the system will have on workload.
Jackie Bawden, head of testing at the QCA, said the test results were needed as part of the trial to highlight any significant differences between teacher assessments and test results.
LEADER 22,Peter Wilby 23