One teacher compared being questioned by inspectors to an interview with famously tenacious BBC journalist Andrew Neil, according to a report from the National Association of Head Teachers (NAHT).
The watchdog’s inspection regime could undermine the government's efforts to improve the teacher recruitment and retention crisis, the union warned.
Ofsted’s new inspection framework, introduced in September, focuses more on the quality of a school’s curriculum rather than exam results but has come under fire from school bosses.
An award-winning headteacher resigned last month after she said an Ofsted inspection last term “extinguished” her passion for the job and left her feeling “powerless”.
The negative impact on teachers has risen under Ofsted’s new inspections, the report from the NAHT says.
Staff have lost breaks and lunchtimes and have been forced to stay late after school in order to satisfy the watchdog’s demands. Some inspection activity continued as late as 9pm, the report suggests.
It says: “The experience of inspection is regularly described by school leaders and their staff as ‘brutal’. Some school leaders have been barred from accompanying and supporting classroom teachers and subject co-ordinators in interviews with inspectors.
“School leaders report that the confidence of recently qualified and experienced teachers who co-ordinate subjects has been shattered, necessitating the counselling of staff.”
Ofsted has announced that schools will get an extra year to bring their curriculum into line with the new inspection framework following criticism from school leaders.
Last month, Lynne Fox, head of Bramhall High School near Stockport, said she would resign in the summer after Ofsted graded the school as “requires improvement” despite improving its GCSE results.
The watchdog criticised the school for allowing pupils to start their GCSEs in Year 9 as they said it “restricted” learning and meant not all pupils benefited from a “high-quality curriculum”.
Leaders of academy chains have also recently accused Ofsted of favouring middle-class children with its new inspection regime.
Paul Whiteman, general secretary of NAHT, said: “Ofsted’s new inspection framework has climbed to the top of many school leaders’ agendas, even above funding in some cases.”
He added: “We have discussed the findings of this report with Ofsted. It is clear that our views differ on how well the inspection framework has performed within the first term.
“Regardless, it is not in the interests of pupils that teachers and school leaders should be subjected to increased and unnecessary workload associated with inspection.
“Nor is it desirable that the high-stakes of inspection should be reinforced, as we know this drives good people from a profession which already struggles to recruit and retain both leaders and teachers.”
An Ofsted spokesperson said: “This framework was the most widely consulted on in Ofsted’s history, and anchored in solid research. So far we’ve carried out over 1,200 full inspections and section 8 inspections of good and non-exempt schools. The feedback we receive continues to be very positive.”
They added: “That said, we recognise that those feeding back to the NAHT have identified areas for improved implementation.”