The number of classes monitored has increased sharply over the past 10 years due to increased school accountability, the introduction of performance pay for teachers and a new induction process for recently-qualified staff.
Teachers in England are facing weekly scrutiny of their lessons as schools and local authorities prepare for the introduction of the Office for Standards in Education's new inspection framework in September. It places increased reliance on self-assessment - something that has figured in school inspections in Wales since last September.
Bunching of observations and negative feedback by heads, inspectors and local authority advisers are undermining teachers' confidence and increasing stress levels, unions warn.
And some local authorities are adopting a more proactive role towards schools they believe may give inspectors cause for concern.
One teacher in Essex has been observed seven times in as many weeks after the council identified his school as giving cause for concern. The National Union of Teachers has responded by issuing guidance to members, calling for heads to agree an annual pattern of lesson observations with staff to avoid "unacceptable" bunching.
Observation for performance management should be limited to one session of no more than one hour per teacher, per year, it said. Regional offices will intervene on behalf of staff where heads refuse to compromise.
John Bangs, NUT head of education, said: "Erratic bunching of classroom observation is highly confusing for teachers. They don't know if they're being observed in a supportive or disciplinary way. There is often no space between them and they are rapidly becoming the bane of teachers' lives."