Supply teachers are turning up the heat in their campaign to have full pay rates reinstated for short-term work.
The Scottish Supply Teachers' Network announced this week it had taken preliminary steps in mounting a legal challenge against the Scottish government.
A firm of solicitors specialising in employment law has taken on its case, although a spokeswoman for the network declined to outline the legal grounds they plan to argue.
The network of supply teachers, who set up a web-based campaign last year, claimed: "The education secretary, Michael Russell, Cosla and the EIS are admitting that there is a massive shortage of supply teachers due to a controversial pay deal."
Last year's teachers' agreement cut experienced teachers' rate of pay to the bottom of the scale if they were doing five days of supply or fewer.
Sumera Tarbard, a Glasgow maths supply teacher and founder of the website Scottishsupplyteachers.com, told TESS she believed things were coming to a head.
Last week, when asked to do supply for two days, she responded that she "did not work fewer than six days" and was immediately reassured she would be paid on the higher rate.
Supply teachers last month refused en masse to provide cover for winter illness in a bid to increase the pressure on the tripartite Scottish Negotiating Committee for Teachers to reinstate higher pay rates.
Ms Tarbard said that following the week-long boycott, many more supply teachers had gained the confidence to refuse to work for less than the higher rate.
Ann Ballinger, general secretary of the Scottish Secondary Teachers' Association, said her union had won victories over two unnamed councils on supply pay.
In both cases, teachers employed on permanent contracts for a day a week were being paid on the reduced rate for short-term supply for additional days worked.
When threatened with legal action, both councils agreed to pay the full rate, said Ms Ballinger.