A survey of field workers for all six teaching unions, carried out by Professor Ted Wragg’s teaching competence project at Exeter University’s school of education, found concern that changes in pension rules meant it was harder for struggling staff to “exit with dignity” by taking early or ill-health retirement.

At the same time, the number of cases of alleged incompetence was increasing, the survey found. Triggers were often new headteachers and approaching school inspections.

Union officials believed more attention should be paid to the causes of members’ problems, such as the burden of change in the education system, conflict at school or problems at home.

In the absence of any official definition of incompetence, some refused to judge members who came to them for help, but most took a view on teachers’ competency - implicitly or explicitly - to help them decide on what advice to give. Some even sat in on lessons to see for themselves, defying official union advice.

The report is the second in an occasional series from the project. The first canvassed heads; others will survey parents, governors, LEAs and teachers.