Educational Support Services has told all its supply teachers it can no longer afford to pay experienced staff higher rates.
Steve Sinnott, NUT assistant general secretary, said the union was now considering the future of their agreement.
From June 14 teachers will be allocated to one of three pay bands instead of one of seven. Those currently at the top of the scale will have their rate cut from pound;115.73 per day to just pound;95.
Less experienced staff will not suffer so much - teachers on point 3 will lose pound;5 a day. The company claims that only 30 per cent of its teachers will be affected.
In a letter to teachers, ESS said that it was responding to "harsh economic reality". "Schools are unwilling to pay for teachers in the top pay bands," it said.
Barbara Charles, customer service director of ESS, blamed a price war between teacher supply agencies. "As a high quality agency managed by teachers we wish to pay proper rates. However, we are losing business to competitors who have no agreement with the unions and take no principled stance on the issue of pay."