A series of strikes can be expected in schools next term, led by Britain's second largest teaching union, the NASUWT, if its members are denied pay rises on the scale for senior staff.
Action is being threatened at three secondary schools in Stoke-on-Trent after walk-outs failed to lead to a settlement.
Teachers at Holden Lane and Trentham held a one-day strikelast week, saying staff were denied a pound;1,100-a-year rise as part of their progression on upper pay spine 3.
Chris Keates, the union's general secretary, said the schools had offered to stage negotiations over the disputes in September, creating unacceptable delays.
Teachers from a third unnamed school have now joined the dispute, Ms Keates said, and unless the negotiation date was brought forward it was "extremely likely" further industrial action could take place.
The union has negotiated settlements in 12 other schools that denied teachers UPS3 rises, she said: "We have a national agreement with the Government on this issue and we intend to see it implemented fully."