Ofsted has lost “almost all credibility with the teaching profession” and needs a new charter to improve the reliability of inspections, a classroom union has claimed.

A motion criticising the watchdog as “no longer just disliked, now disdained” was carried at ATL’s annual conference, which got under way in Manchester this morning.

This is just the latest criticism the watchdog has faced in recent weeks. Last month, the Policy Exchange think-tank called on the watchdog to scrap “unreliable” lesson observations and cut its use of externally-contracted inspectors.

Several days later, chief inspector Sir Michael Wilshaw announced plans to scrap the use of additional inspectors contracted from private firms and scale down inspections for schools rated good.

But ATL delegates today called for a charter with “robust guarantees” that Ofsted  teams would “always include a majority of experienced and currently serving teachers and school leaders”.

“Unsubstantiated” complaints against schools or teachers should be “wholly disregarded” by the watchdog, delegates agreed, with individuals able to make confidential complaints about the conduct of inspectors.

Simon Clarkson, a teacher from Leicestershire, told the conference that Ofsted was “distorting the fabric of education” and that the watchdog was “broken”.

“Because [Ofsted’s verdicts] are so subjective, because they are so fallible and because your job, the school’s future and your leadership team’s future depends on what they say, our headteachers now routinely demand the impossible,” he said.

“I am a middle leader. I am told to ask for more and more from staff who are exhausted and have less and less to give. I see solid, dependable colleagues who I know are excellent teachers in tears ... We need to stop the madness. We need to stop the Alice in Wonderland management of our schools and ... we need to fix the distorted looking glass that is Ofsted, that is distorting the way we teach. Ofsted is broken, it needs fixing.”

Pat Kyrou, a teacher from Essex, said: “Ofsted, it is not your role to terrify teachers. It is not your role to force teachers to teach in a way that is less beneficial to the pupils.”

In a survey published by ATL today, more than a third (38 per cent) of school and college staff said they had noticed a rise in mental health problems among colleagues in the past two years, with Ofsted cited as a commoen cause of stress.