Headteachers are expected to heavily criticise Ofsted chief Sir Michael Wilshaw, suggesting his "negative rhetoric" is creating a "culture of fear" in schools.
Delegates at the National Association of Head Teachers (NAHT) annual conference in Harrogate are due to debate an emergency motion warning that they are "saddened and angered" by Sir Michael's approach to school inspections.
In what could be seen as a personal attack on the chief inspector, the motion says that as a former head himself, Sir Michael should understand that to get best out of children and school staff they need to be both challenged and supported.
Sir Michael, who became chief inspector in January, was previously executive head of Mossbourne Academy in Hackney, east London. The full text of the motion says: "This conference is both saddened and angered by the approach taken by the current HMCI (Her Majesty's Chief Inspector of Schools).
"We deplore his negative rhetoric which is creating a culture of fear in schools. We would have expected him, as a former fellow school leader, to understand that to get the best out of children and staff in schools, we need to both challenge and support. We call on national executive to pursue whatever action it deems appropriate to defend our profession."
The move represents a further deterioration of relations between the union and Ofsted, and comes just days after the NAHT raised concerns over the variable quality of inspections and the watchdog's planned changes to the inspection system.
A poll of more than 2,000 school leaders, conducted by the union, found that almost half (45.3%) believe that Ofsted makes no contribution to, or actively prevents, standards being raised. And nine in 10 (89.9%) are either unhappy or very unhappy about the tone and content of recent announcements by the watchdog.
Among others, Ofsted has recently announced plans, that are currently out for consultation, to introduce no-notice inspections for all schools, and to scrap the "satisfactory" rating and replace it with "requires improvement".
An Ofsted spokeswoman said: "Ofsted has been listening to the views of headteachers, teacher and parents about its proposed changes to school inspections and will announce the results of its consultation at the end of the month. The intention is to work closely with good heads as they drive improvement in their schools."
On Saturday, Education Secretary Michael Gove appeared to offer an olive branch to school leaders as he signalled a U-turn over plans for schools to face unannounced inspections, which were expected to be introduced this autumn.