Ofsted is under attack for refusing to join the Government and the Qualifications and Curriculum Development Agency (QCDA) in reassuring teachers that they do not have to use a controversial pupil assessment scheme.
Teaching unions fear Assessing Pupils' Progress (APP), which involves keeping files on pupils and regularly assessing their progress against a series of detailed criteria, will burden their members with extra work.
The scheme has never been a statutory requirement. But since it was first made available to schools last year a succession of official documents have implied that all schools must use it and further pressure has been applied by local authorities.
Teachers' leaders believe that Ofsted can help relieve that pressure because it is the schools watchdog's inspections that are used to enforce what happens in schools.
But at a recent meeting Ofsted refused to sign a statement that said the use of APP was "voluntary".
The watchdog reportedly told teaching unions that it did not feel it was appropriate to do so, despite the fact that the Department for Children, Schools and Families and the QCDA have both signed the document.
John Bangs, head of education at teaching union the NUT, said: "We want reassurance from Ofsted that it will not be part of getting schools to obey a rule that does not exist.
"There is still a real concern about the excessive workload that could be caused by APP.
"Local authorities have really got the bit between their teeth on this and will expect inspections to be their lever of enforcement. That is why we need Ofsted on board."
He said the watchdog's position was "totally and utterly inconsistent" because it had signed joint guidance in 2004 that relieved teachers from the pressure threatened by lesson planning, and in 1998 had said that numeracy and literacy strategies were not a requirement.
Ofsted could not say why it felt it was inappropriate to sign the statement.
But a spokesman said: "When inspectors look at schools' approaches to assessment they do not specify particular methods but look at the effectiveness of the schools' work in this."
The 2004 joint statement said that APP should use evidence from day-to-day teaching and learning and not involve any special assessment activities.
It also said that use of the scheme should be subject to discussion and consultation with staff before it is introduced in schools and that it works best when used no more than twice or three times a year.
Previous official briefings on APP could have led schools to believe that it was a statutory requirement.
A Qualifications and Curriculum Agency (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 then embed APP.
April's Rose review into the primary curriculum said: "APP should be widely in place across all year groups by the time the new curriculum in implemented in 2011."
In May, the Government's "experts' group" on assessment said the Government should continue to promote the use of APP in "all primary and secondary schools".