Eamonn O'Kane, the union's general secretary, believes teachers will find it more difficult to cross the threshold from the main to the upper pay scales once the changes are implemented.
"We are pushing very hard for the Government to continue with the present system where threshold pay awards are financed through a ring-fenced fund," he said. "We understand that they are considering phasing this out, and for the pay awards to be absorbed into school budgets.
"A headteacher will be more reluctant to award a threshold pay rise if it has to come out of his own budget, rather than if the rise is to be financed from a separate fund as is the case now."
As many as 60,000 teachers have this year reached the top of the main pay scale and have become eligible to cross over to the upper scale in a performance-related pay scheme.
Last September, a new six-point main scale came into force, replacing the old nine-point scale, which means teachers are now reaching the top of the main scale earlier.
The threshold was introduced in September 2000 enabling qualified classroom teachers to progress to a higher salary range, if they can successfully meet eight national standards.
To date, around 95 per cent of teachers who apply are successful. In the first round of threshold payments in June 2000, 201,000 teachers applied to cross the threshold. The following year, the last for which figures are available, there were 31,200 applicants.
Teachers who have now reached M6 on the main pay scale who are seeking promotion to the upper scale in September 2004 should submit their applications to their headteacher or service manager on or before December 1.
More than 100,000 teachers will be receiving extra cash in their pay packets this month as they move up a point on the pay scales applicable to them. This year's pay award, introduced on April 1, represented a 2.9 per cent pay increase in line with inflation for most teachers.
The exception was for teachers in inner London who were for the first time placed on a separate pay scale of between pound;21,000 and pound;30,000, and an upper scale of between pound;34,000 and pound;39,000.
The new pay scale for the first time enables a classroom teacher to earn in excess of pound;50,000 a year without taking on extra management responsibilities. The pound;50,000 salary level can be achieved by an advanced skills teacher working in inner London who is on at least point 24 on the 27-point AST scale.