The threat follows the publication of an Order in Parliament on Wednesday which brings the new pay system into effect. From next week teachers in England will discover whether they have successfully crossed the threshold, which guarantees a pound;2,000 pay rise and access to a higher pay scale of up to pound;30,000.
External assessors will begin school visits this week, although most will occur in the spring term.
But the National Union of Teachers believes that new regulations will infringe teachers' rights by denying them the chance to take their case to a tribunal if they are refused the payments.
The NUT halted the process in July with a High Court victory that ruled that Education Secretary David Blunkett had acted unlawfully by failing to consult properly on the proposals. The Government then agreed to have an appeals procedure.
The union hopes its latest concerns can be resolved by discussion but would not rule out a legal challenge. It argues that any legal action over appeal procedures will not delay payments for teachers who have successfully applied.
Doug McAvoy, NUT general secretary, said: "Teachers who believe that they have been subject to discrimination in relation to their threshold applications should have the right of access to employment tribunals. The denial of that right is a denial of their fundamental rights as employees."
The National Association of Schoolmasters Unio of Women Teachers also opposes the review system, but baulked at action which risked delaying payments.
However, the union said it would be prepared to take individual cases to court to establish the right to a full appeal.
Meanwhile, the Government has put back its performance-management deadline for headteachers to be appraised by their governors. All 24,000 school boards in England now have until April 6, not the end of December, to meet with their heads.
Performance-management training for governors began this term, but many have complained of delays in getting an external adviser from contractor Cambridge Education Associates.
At this week's National Governors' Council's conference, Jo Wakeham, from Cornwall, said a survey of 65 local schools had revealed governors waiting 15 minutes on hold when calling CEA and advisers failing to arrive for meetings.
A company spokeswoman said: "The number of schools arranging for external advisers is increasing daily and, from our point of view, the process is going very well."
Teachers in England will be assessed on original applications
Threshold standards remain
Teachers in Wales have until March 16, 2001, to submit bids
Heads can make a complaint about external assessors
Teachers who feel they have been wrongly assessed will be able to seek a review
All successful applicants will receive their pay increases backdated to September 1, 2000
Most teachers should get the money by Easter