Delegates of the extreme left inflicted a series of defeats on the NUT's leadership at this year's Blackpool conference so serious that they threaten the union's future, according Doug McAvoy, the union's general secretary.
The power of the militants has been growing in recent years. This year extremists have succeeded in co-ordinating their conference campaign more effectively than ever before.
Their rapid rise within the union has been made possible by the inertia of moderate teachers. Most have been overwhelmed by the burden of changes thrust on them by the Government in the last seven years. They are too demoralised and weary to turn up to evening union meetings.
Meetings that used to be attended by several hundred members in big cities such as Birmingham now have difficulty in mustering more than a few dozen enthusiasts. They are dominated by political groups that are more interested in discussing international affairs than issues which concern classroom teachers such as testing and class size.
Ever since the early Seventies the NUT conference has had a proportion of delegates on the extreme left. A small number are Socialist Worker Party members, but most come from the hard left of the Labour Party. This year the extreme left accounted for around one-third of delegates.
There are two main pressure groups - the SWP-inspired Socialist Teachers Alliance and the Campaign for a Fighting & Democratic Trade Union. The latter claims to include people of all political persuasions but is mainly an umbrella group for left-wingers.
Moderate conference delegates believe they detected a new and ugly element in the extreme left at the demonstration against David Blunkett, Labour's education spokesman, at the weekend.
The militants have traditionally been vociferous at conference but have never before resorted to the tactics that led to Blunkett being barricaded in a room while demonstrators howled outside.
Most of the SWP members in the Blunkett protest appeared to be classroom teachers but moderate delegates say some of the most determined activists spend little time teaching because they have union posts and are therefore allowed to spend several days each week away from the classroom. They are comparatively isolated from the views and concerns of ordinary teachers.
The SWP members at the conference were highly organised. Every morning they produced professional-looking leaflets that were handed out to delegates as they arrived. They held fringe meetings for delegates and constantly met among themselves to decide tactics.
Party members who are not delegates are often given observer status by their local branches, sell Socialist Worker newspapers and sit in the gallery for debates.
The party's interest in the NUT is not surprising. It is a middle-class union whose rules make a takeover a real possibility. Unlike the other two main unions, the National Association of Schoolmasters/Union of Women Teachers and the Association of Teachers and Lecturers, the NUT's conference is a policy-making body.
With 232,000 members the NUT, the biggest and oldest teaching union, is an obvious prize.
Outside the union, militant left-wingers have been active in trying to infiltrate the new parents groups which have sprung up to fight the cuts.
Earlier this term the NUT leadership organised a national lobby of Parliament in the same week as the parents' group Fighting Against Cuts in Education (FACE) organised another demonstration. The NUT leadership accused militant members of trying to take over the parents' campaign.
In fact, though SWP members attended the demonstration, moderate parents and teachers are still firmly in control of the group. Meanwhile, as Mr McAvoy recognised yesterday, the NUT is in danger of losing thousands of members unless it can find a way of kerbing the extremists' influence.