Teachers are expected to raise fresh concerns about school inspections, as one union leader suggested there should be a public inquiry into Ofsted's role.
The schools' watchdog has become increasingly "politicised", according to a poll by the NASUWT union, while inspections themselves are causing teachers "significant" levels of stress.
Ofsted is facing increasing pressure from teaching unions, who are ramping up calls this weekend for chief inspector Sir Michael Wilshaw to quit, and for the inspectorate to be axed.
Both the NASUWT and the National Union of Teachers (NUT) are due to debate motions on Ofsted.
Speaking ahead of the NASUWT's conference in Bournemouth, general secretary Chris Keates said: "The easiest way to describe it is that there's been a politicisation of the inspection service.
"Schools have to be held accountable, the deep concern about Ofsted is it's no longer holding schools to account in the public interest, it has become the hit squad of the Secretary of State to implement his policies."
Ms Keates said that the union believes that the current role of Ofsted need to be examined by Parliament.
"I think it needs a parliamentary debate. It's almost serious enough to have a public inquiry."
A poll of around 2,500 NASUWT members found that 95% think that Ofsted is acting in the interests of politicians, while 81.4% said the current inspection regime unfairly undermines public confidence in the education system. More than nine in 10 (91.7%) said that preparing for inspection is a "significant contributor" to excessive workload, and 71.2% said inspections lead to significant levels of stress.
An Ofsted spokeswoman insisted that the inspectorate has not been politicised. "Ofsted does not have a preferred model for schools; we are independent and are not about furthering any political agenda," she said.