The target class size for primary schools is 18 pupils
Scotland's largest teaching union, the EIS, is to debate taking industrial action over class sizes.
The EIS proposed a 20 pupil cap 12 months ago. Now three branches of the union are to urge members to increase the pressure to meet that target.
At the union's annual conference, the Glasgow branch will propose industrial action over the class size issue.
Scotland's local authorities described the move as "disappointing and counter productive".
The Glasgow branch has been joined in its campaign by South Lanarkshire and East Renfrewshire.
The East Renfrewshire branch will use the Perth conference, in two weeks' time, to call for small classes to be reinforced by statute.
Scotland's local authority umbrella body, Cosla responded to what it called "the EIS threat".
Cllr Isabel Hutton, Cosla's Education Spokesperson said: "This is disappointing and counter productive.
"The EIS knows full well that pressures on all local authority budgets are increasingly severe but, despite this, many councils are managing to protect education budgets and to increase them in several cases.
"We also need to clarify the incorrect impression on the broadcast media this morning that local authorities received additional funding to help deliver reduced class sizes in P1-P3 - this is simply not the case.
"There are no resources in the settlement to reduce class sizes."
In the last year the SNP government has made some limited progress in delivering its manifesto pledge of no more than 18 to a class in early primary.
In May 2008, the union called for class sizes to be brought down over time to a maximum of 20 in primaries and secondaries.
It submitted a petition to the Scottish Parliament, which was signed by almost 80,000 people, calling for "significant reductions" in class sizes over four years.
Members of Holyrood's education, lifelong learning and culture committee was told there was "significant" evidence that cutting class sizes benefited students.