South Ayrshire and East Dunbartonshire said last week they would continue to use their own arrangements, a decision already taken by Glasgow, South Lanarkshire and Renfrewshire.
The authorities' reactions were condemned as "unsustainable" by Ian Penman, chairman of the London-based TimePlan organisation. Mr Penman confirmed that the company was taking legal advice and added: "I would advise the authorities to do the same before it is too late. Restraint of trade is not just a question of United Kingdom law but it is a question of European law as well, and a very expensive question."
Mr Penman said TimePlan was not offering its service to local authorities but to headteachers and teachers "who are getting a pretty poor deal from the authorities, according to them".
He added: "I have just come back from the National Association of Head Teachers' conference in Torquay. We have established a wonderful relationship with them and the reason is that their members are happy to use our service to mutual and maximum advantage.
"I do not see why the Scottish local authorities cannot acknowledge that instead of bullying schools into a position for ideological reasons."
The authorities' main concern, however, is the appearance of teachers in their schools over whom they have no control as employers. A report to South Ayrshire's education committee last week said the council had to be satisfied about "the appropriateness of staff who have unrestricted access to pupils".
Contracting out teacher supply "would leave the authority exposed should any individual employed by an independent agency fall below acceptable standards".
East Dunbartonshire said it "would wish to retain control over the registration and deployment of supply teachers in order to ensure the effective delivery of council policies".
But the council appeared to leave the door open by acknowledging that schools might find it an advantage using an agency to ease a subject shortage or to open up wider job opportunities for staff. The value of the teacher supply business in East Dunbartonshire alone is put at more than Pounds 1 million.
TimePlan is no stranger to legal action, winning a High Court battle in London last year with the National Union of Teachers over accusations that it undercut rates of pay south of the border.