The Scottish Government has been accused of dropping a key manifesto commitment on cutting class sizes.
Ms Hyslop said 'flexibility' was needed in reducing class sizes
The SNP had pledged to reduce class sizes to 18 in the first three years of primary school.
Education Secretary Fiona Hyslop called for flexibility, adding that she would not want to disrupt a P1 class of 25 just to bring it down to 18.
A government spokeswoman added that it remained committed to driving class sizes down to 18.
Ms Hyslop's comments in an interview with the Times Educational Supplement Scotland were immediately seized upon by the opposition who branded it evidence of a "broken promise".
Labour leader Wendy Alexander said: "The SNP have finally admitted that they won't now reduce class sizes in primary one, two and three.
"A lot of people across Scotland voted SNP because of this and yet less than six months into this SNP government, their education secretary has admitted it won't happen.
"It is another SNP broken promise."
The Labour leader called on First Minister Alex Salmond to explain why the party had gone back on the issue after he "guaranteed" class sizes would be reduced in the Scottish Parliament.
The Scottish Government spokeswoman said: "The cabinet secretary has made clear all along that she wants councils to take a pragmatic, common-sense approach to reducing class sizes, with the educational benefit for children being the most important aspect."
Tory education spokeswoman Liz Smith said it proved that the Nationalists did not think through a key election pledge.
"Today's position is doubly embarrassing for the SNP," she said.
"Not only has Fiona Hyslop today broken a pre-election pledge, but it is a pre-election pledge that should never have been made in the first place, as it is unrealistic, undesirable and unworkable."
Lib Dem leader Nicol Stephen said it showed that the Nationalists failed to plan or cost their biggest policies.
"The cracks are now starting to appear in this SNP government, with one broken promise after another," he said.