One in five teachers was observed more than six times last year, according to a survey of almost 7,000 teachers by the NASUWT. It is pressing the Government to set an annual limit on the number of lessons that can be monitored by senior staff, local authority advisers and inspectors.
The union says teachers should be given clear written feedback after each observation, which the survey suggests occurs in only two in five cases.
More than a third of the 6,887 teachers who responded to the online survey said observations were not done fairly and consistently. A similar number said they had no choice about when the observation would take place. One in 20 was given no notice.
Chris Keates, NASUWT general secretary, said: "The lack of feedback in some schools combined with no agreed protocol and a failure to consult on when observations will take place demonstrates some teachers are being subjected to excessive monitoring rather than professional observation.
"The union is seeking an end to excessive, punitive monitoring and the introduction of an entitlement for all teachers to supportive, developmental classroom observation."
Carol Adams, chief executive of the General Teaching Council, said the NASUWT was right to criticise unplanned observations but that schools were right to monitor weak teachers regularly.
She said: "Teachers value observation as a form of professional development when it is well structured, with agreed goals and feedback. Observation for monitoring purposes is necessary, but it needs to include a clearly understood purpose and outcome."