Members of the Scottish Secondary Teachers' Association were originally expected to hold a formal ballot on industrial action when they returned to school in mid-August. The union claims the initiative is under- resourced and teachers are not yet ready to move to full implementation.
An indicative ballot last month on industrial action received 95 per cent backing, but only 28 per cent of members responded. The low response rate was described as a "rebuff" to the union's plans by Education Secretary Michael Russell.
Now SSTA president Peter Wright has decided that any formal ballot must first be endorsed by the union's executive committee, which will not meet until August 29 - effectively delaying the prospect of a full ballot by at least two weeks.
And Ann Ballinger, SSTA general secretary, has warned that for a "work to contract" to go ahead, it would need to be backed by a majority of the union's members.
She said: "The executive will decide what the parameters of a ballot should be, what response rate we would need before we could call it a reasonable response, and what percentage in favour we would need to call for a work to contract."
Ms Ballinger added that teachers should be "working to contract" in any case.
"If this ballot goes ahead, we will not be asking teachers to do anything they are not supposed to be doing anyway, which is working 35 hours a week," she said.
If teachers were to work to contract, there would be the following impacts, she predicted: there would be a longer gap between work being handed in by pupils and teachers returning the work corrected; homework clubs and lunchtime activities would be hit; and less new material would be produced.
"Virtually all new material produced is done outwith the 35-hour working week," she said.