Teachers are being reassured that a scheme designed to help them monitor pupil development, which unions fear will create extra workload, is not compulsory.
Since Assessing Pupils' Progress (APP) was made available last year, a string of official documents have implied that all schools must use it.
In fact, APP, which involves keeping files on pupils and regularly assessing their progress against detailed criteria, has never been a statutory requirement.
This is now clarified in a briefing note for teachers from the Qualifications and Curriculum Authority, which developed APP. It states that "its use is voluntary".
The document follows pressure from the NUT which, along with other teachers' unions, wants to prevent the scheme causing excessive workload.
Christine Blower, NUT general secretary, said: "There may be merit in APP, but only if it's voluntary and optional. Its use should be determined by the professional judgment of teachers."
The QCA briefing, to be distributed by the unions, states that APP should use evidence from day-to-day teaching and learning and not involve any special assessment activities. It also says the scheme should be subject to consultation with staff before it is introduced, and that it works best when used no more than three times a year.
Previous APP briefings could have led some schools to believe it is a statutory requirement. A QCA guide published last year said: "The expectation is that by 2011 all schools will have embedded the APP approach."
In June 2008, the Government's Assessment for Learning strategy said its objective for 200809 was to make sure every school was introduced to, and offered training for, APP in reading, writing and maths.
Objectives for 200910 and 201011 were to offer direct support to schools that had not managed to establish and embed APP.
The Rose review of the primary curriculum in April said "APP should be widely in place across all year groups by the time the new curriculum is implemented in 2011".
Last month, the Government's "experts group" on assessment said officials should continue to promote the use of APP in "all primary and secondary schools".
John Bangs, head of education at the NUT, said there had also been pressure on schools from local authorities to implement APP.
"We are in a grey area of intense government advice and quasi requirements that can have a very persuasive hold on schools," he said. "But something is either a statutory requirement or it's not."
Mr Bangs said APP would not be implemented properly if schools felt they had no choice. "It won't be integrated into what they are already doing but as an add-on that creates extra workload," he said.
"We are not against APP. We are against it being imposed."
APP is intended to give teachers a structured way to introduce Assessment for Learning (AfL). They are asked to consider evidence from their day-to- day interactions with pupils demonstrating what they can understand and do - for example, pieces of writing, oral presentations - or observations of pupils' behaviour.
The QCA guidelines aim to help teachers assess this evidence in relation to national curriculum levels.
But some experts, such as Professor Gordon Stobart from London University's Institute of Education, fear "leveling every piece of work" misses the whole point of AfL's focus on actual learning.