A survey of 1,600 teachers for the Department for Education has found that all but 3 per cent belong to a union.
Most said they had joined a teaching union to get support if they had a problem at work. Only 5 per cent of primary and 7 per cent of secondary teachers said it was because unions campaigned on issues that mattered to them.
Of the 3 per cent who were not part of a union, half had been members in the past.
The survey, carried out by the National Foundation for Educational Research, also found that six out of 10 teachers had used union services in the past 12 months.
More than three-quarters of respondents said there were no members of the teaching staff at their school who worked full-time on activities connected with their role as a union representative.
Union services used in the past 12 months
Advice from union website - 35%
Training - 19%
Advice from helpline - 17%
Model policies - 16%
Discounted services - 13%
Face-to-face support with an issue at work - 13%
I have not used union services - 38%
Advice from a legal professional - 2%
Other - 6%.