They backed an amendment instructing the executive to ballot for action before next year's conference should negotiations this winter fail.
The Government stalled strike threats over its planned pension reforms by agreeing to negotiate with public-sector unions.
But Keith Anderson, of Suffolk, said the union had to send a clear message.
"Trade unions should not be in a position of negotiating for a shorter life for their members," he said. Dave Wilkinson, of the union's national executive, was laughed at when he urged members to trust their leaders'
Steve Sinnott, NUT general secretary, said he was "absolutely certain" the Government's offer of talks was genuine. But, he said: "If the Government fails to deliver I have absolutely no doubt that the anger that there is among teachers would be increased."
All but one NUT delegate voted to threaten a return to industrial action unless the Government withdrew all proposals to worsen teacher pensions.
Stefan Simms, from Enfield, said to cheers: "Let's be clear what pensions are. They are deferred wages. Prescott, Blair that is my money. Keep your grubby hands off it."