Tensions between England's headteachers and Ofsted are developing into a crisis after school leadership unions said they had no confidence in the watchdog and would hold strike ballots unless "significant wellbeing issues" caused by Ofsted's inspections regime were fixed.

The Association of School and College Leaders (ASCL), which includes senior leaders in many secondary schools, described Ofsted's inspection system as "an overbearing mess" and urged the government to pause inspections to stop teachers leaving the profession.

A spokesperson for ASCL said that "if change is not forthcoming" the union would seek to hold an indicative ballot on industrial action.

The National Association of Head Teachers (NAHT) said it would also consider steps including strike action at a meeting of its leadership next week.

The combative moves come after a survey published by Ofsted showed 88% of schools reported that recent inspections had negatively affected staff wellbeing.

The Guardian reported last week that Lucy Powell, the education secretary, wants Ofsted to "go back to the drawing board" and overhaul the framework introduced last year. The new framework was intended to lower the strain and workload caused by inspections, but many heads say the opposite has happened.

In response to ASCL's announcement, a spokesperson for the Department for Education said: "Inspection and accountability are vital, but they must reflect the reality of what is happening in schools, recognise the context they are working in, and support improvement rather than add to the pressures they already face.

"That's why we are working with Ofsted and school leaders to ensure accountability is fair, proportionate and focused on helping every school get better, so every child gets the education they deserve." An Ofsted spokesperson said: "We have worked hard to reduce the pressure of inspection in recent years, but we know workload and wellbeing remain a challenge.

"We are working with the Department for Education on this and we will keep listening to feedback from professionals as we continue to improve what we do." Hostility towards Ofsted has been heightened since the death of Ruth Perry, a popular headteacher in Reading. In 2023 a coroner found that an Ofsted inspection contributed to Perry's mental health deterioration and her death. Labour pledged to reform school inspections in its 2024 election manifesto.

But ASCL said it had "seen no meaningful progress on the part of Ofsted to address significant wellbeing issues caused by inspections, and that it is increasingly concerned about the welfare and morale of members".

The motion of no confidence in Ofsted passed by ASCL's executive stated: "We fear that the disproportionate stress and anxiety exerted by the inspection system will lead to school leaders deciding to leave their posts with a consequent impact on schools, children and young people."

Pepe Di' Iasio, ASCL's general secretary, said: " Our repeated attempts to resolve our concerns through discussion and negotiation have hit a brick wall … The current system is not fit for purpose. It is an overbearing mess and urgently needs to be reviewed."

Paul Whiteman, the general secretary of the NAHT, said: "Our deep concerns about this Ofsted framework and the impact it is having on professionals are longstanding. Our executive committee will be meeting at the end of next week and debating the next steps NAHT will take. We fully expect that debate to include the option of pursuing industrial action."