Doug McAvoy, general secretary, said the Government has not moved far enough to meet the union's concerns, despite making some concessions to its original pay proposals set out in the Green Paper. "Our campaign continues," he said.
From September all schools must introduce new performance management procedures. But the union has warned them not to base their systems on the framework and criteria used by inspectors.
"Performance schedules for classroom observation which involve checklists have the potential for creating bureaucratic burdens and for undermining rather than supporting teaching staff.
"Performance management procedures must be agreed on a collective basis between head teachers and their staff after full consultation," he said.
The Government has delayed the introduction of appraisal linked to pay until September 2000 but intends to run a number of pilots in the autumn. Any pilot based on the Green Paper will be boycotted by NUT members.
Mr McAvoy said the union will take a decision on whether to boycott threshold assessment when the Government publishes the criteria later this year.
This week Charles Clarke, schools minister, announced that pound;22 million will be earmarked for training staff to implement the new pay system. The Green Paper proposes that good teachers will be able to pass a performance threshold, giving them a pound;2,000 rise, and will go onto a performance related pay scale with a potential salary of pound;35,000.
An NUT survey found its members feared the prospect of their pay being determined by their headteacher. Mr Clarke said he was confident the new pay system would not involve cronyism, and the training would ensure effective teacher assessment.