The National Union of Teachers has warned it will ballot members for industrial action, including strikes, if headteachers try to persuade inspectors to downgrade teachers for disciplinary purposes.
Doug McAvoy, the union's general secretary, said he was concerned about remarks from the Secondary Heads Association suggesting that their members are being undermined by inspectors failing to award low grades.
Teachers are graded from 1 to 7 under the Office for Standards in Education's new code of practice.
Mr McAvoy said: "Heads are responsible for the management of their staff.
"They are in the school the whole time - OFSTED inspectors can merely provide a snapshot of the work of a school. They should not attempt to evade their management responsibilities by trying to use that snapshot improperly by persuading OFSTED inspectors to take over their management duties and do their work for them.
"If a head acts in this way the union will have no hesitation in balloting members for action, to include if necessary strike action, to prevent headteachers shrugging off their duties and misusing inspections."
Mr McAvoy said headteachers concerned about members of staff should try to help them, not wait for a six-yearly inspection before addressing the problem.
John Sutton, general secretary of SHA, said Mr McAvoy had misunderstood.
"We feel undermined because it is difficult for us to take action against a weak member of staff if inspectors give them a reasonable grade.
"We do not want OFSTED inspectors to take over our duties - quite the contrary."
An OFSTED spokesman said the grades were meant to be used only as a guide for headteachers. OFSTED has admitted that there has been heavy use of grade 5 which assesses teachers as poor but does not warrant further action.
Sheila Russell, president of the National Association of Inspectors, Advisers and Consultants, said she believed the grading system was unfair and unworkable.