The Equal Opportunities Commission has decided to become involved in the case for the first time and will share the costs of the action. But Craig Duncan, the association's acting general secretary, said: "We are talking about a large sum of money, which could be double the costs of the three previous stages. "
The union would not divulge the cost of appealing against the ruling by the Court of Session in July that Strathclyde and its 12 successor councils were not in breach of the 1970 Equal Pay Act by requiring nine union members to carry out the work of principal teachers without the extra pay.
Significantly, the key decision to press on with the case was taken by the finance committee. The union had a Pounds 10,500 deficit last year.
The SSTA won earlier hearings before an industrial tribunal and the Employment Appeals Tribunal and the stakes are high on both sides. Up to 500 teachers could make similar claims and local authorities across Scotland face a bill of up to Pounds 20 million in back pay.
Mr Duncan said: "The counsel's opinion we received was that there was enough in the Court of Session's judgment that could be challenged for an appeal to the House of Lords to be worth while."
Strathclyde and its successors have consistently maintained that there was never any intention to discriminate against women teachers on sex grounds and that resort to equal pay legislation is unjustified. The teachers say they were carrying out the work of principal teachers and used male comparators to prove their case.
Three of the SSTA members head departments at Vale of Leven Academy in West Dunbartonshire, all paid as senior teachers which means they earn almost Pounds 4,000 less than principal teachers. Kathleen Wallace, the union's district secretary, is in charge of religious education; Eraine Black looks after learning difficulties; and Shona Miller's remit is computing.
The association's decision to stick by its members will embarrass Unison, the local government union, which suddenly withdrew legal support from three heads of pre-five centres in the west of Scotland on the Monday following the union's Court of Session defeat (TESS, August 9).
The heads were claiming parity with nursery heads, which could have cost councils in the former Strathclyde area Pounds 5 million.