The Scottish government is to bring in legislation to cap class sizes in primary one at 25 pupils.
Education Secretary Fiona Hyslop denied the plan was a weakening of a manifesto promise to reduce early primary classes sizes to 18.
She said the 25-pupil limit would provide a "backstop" for local authorities facing legal challenges.
Scottish Labour said the SNP's 18-pupil policy was being "ripped up and thrown away".
Many local authorities have claimed they do not have enough money, staff and classrooms to deliver what the government wants.
Parents have won legal test cases allowing them to send their children to the school of their choice, even though councils have told them classes were full.
In England and Wales, there is a legal limit of 30 pupils in the first three years of primary education.
Ms Hyslop said a class size review would see how the rules could be made "more coherent".
Class size limit for P1
She said local authorities had changed their mind about the need for legislation to back up the policy.
She told BBC Radio's Good Morning Scotland programme: "What we are doing is providing the legislation to support smaller class sizes. We think local pupils going to local schools should benefit from smaller class sizes."
She insisted that good progress was being made towards reducing class sizes, but declined to say when the promise of no more than 18 pupils in the first three years of primary might be met.
"We have provided the resources to allow local government to make year-on-year progress in delivering smaller class sizes. The manifesto will be delivered as we have agreed with local government by making year-on-year progress," she said.
However, she also acknowledged that the economic recession was placing pressure on councils.
Labour education spokeswoman Rhona Brankin said: "There was no mention of legislation on class sizes in the recent programme for government but the cull of the 18 class size pledge has been on the cards for months.
"It's now thought that the policy will be ripped up and thrown away."
The Scottish Conservatives said the SNP had misled voters with its manifesto promises.
Tory schools spokeswoman Liz Smith said: "It all adds up to one thing - the SNP has misled parents and, as with the pledge to dump student debt, was only interested in conning the electorate.
"Regardless of whether or not this much needed U-turn is carried out, Alex Salmond and Fiona Hyslop should be ashamed of themselves."
Liberal Democrat education spokeswoman Margaret Smith said the SNP had admitted defeat over its flagship education policy.
She said: "SNP ministers have failed to deliver the legislation, the funding, or the teachers for councils to reduce class sizes across Scotland in line with this pledge.
"It is another example of the SNP centralising the policy but localising the blame."
Ronnie Smith, of the EIS teaching union, said the cap on P1 classroom numbers was a "step in the right direction" and welcomed the review of regulations.
He said: "The EIS has drawn attention in the past to the current 'patchwork quilt' of class size arrangements and the means of promulgating such arrangements.
"A review offers an opportunity to provide a coherent approach to class size maxima and the EIS will play its part in this review."
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