Councils have been unable to cap class sizes at a reduced level Plans to bring in a legal limit of 25 pupils in primary one are expected to be announced later by the Scottish government.
The proposed cap is some way short of an SNP manifesto promise, which pledged classes in the first three years of primary would be capped at 18 pupils.
The proposed new limit would have clout as it would be set in law.
Previous plans to reduce class sizes have failed because schools could not legally say classes were full.
Families refused a place were often successful when they went to court to appeal against decision not to allow their child to attend on the basis of class sizes.
Under current rules each teacher can have up to 30 pupils in their class.
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