The union has seen an increase of more than 20,000 members, according to figures for 1996 submitted to the Certification Office.
Almost a third of the increase is due to inclusion, this year, after auditors' advice, of newly-qualified teachers in their first term, for whom membership is free. NUT leaders claim the increase in membership, from 192,009 in 1995 to 212,928 last year, is partly due to greater membership involvement in decision-making and better services.
But the union also suggests the rise is a "mopping up" of previously non-unionised teachers, particularly as a reaction to the threat of inspections by the Office for Standards in Education.
Malcolm Horne, a former NUT president and current secretary of the union's Brent association, said teachers were choosing the NUT because of its trenchant attitude to the school inspection regime and the "very good advice" it gave members on the issue.
In an apparent swipe at the NASUWT, the second largest teachers' union, whose membership rose from 157,146, Arthur Jarman, NUT membership secretary, said: "The increase has been generated not by gimmicks and publicity-seeking campaigns but by providing a service which represents the interests of teachers."
The issue of teaching-union membership figures always sparks debate because of the varying ways they collate figures.
The latest membership figures for the other two classroom unions are: Association of Teachers and Lecturers 130,339 (1995), 131,799 (1996); Professional Association of Teachers 42,595 (1995), 40,178 (1996).