Minutes of a meeting in July between the education deans of Scotland's universities and civil servants, led by Donald Henderson, show that official advice was: "The scale of the commitment does not allow it to be delivered in the lifetime of a parliament."
Mr Henderson, then head of the schools directorate, also told the meeting that, with the scale of the plans to reduce P1-3 classes to 18 and the numbers required to replace retired teachers, "the commitments will take 8-10 years to achieve".
Details of the meeting, revealed under a Freedom of Information request, have unleashed an attack by the Labour Party. Rhona Brankin, Labour's shadow education secretary, said: "The fact that SNP ministers were told last July that their plans were undeliverable speaks volumes about everything that they said at the time and repeatedly since ... To state in the concordat that councils must 'as quickly as possible' deliver class sizes of 18 is disingenuous, given that the SNP were explicitly told by the Government's own advisers that it was not deliverable within this spending review."
Meeting notes also show officials believed that to deliver the commitment would require an extra 4,000 student teachers and the doubling of places available on BEd courses.
But officials also warned: "A judgment has to be made as to at what level of intake there starts to be a risk to professional quality."
They suggested one solution would be to re-balance the postgraduate and BEd intakes, which had been "out of kilter" because the previous Scottish Executive's class-size reduction commitments had been delivered through the one-year postgraduate route.
A Government spokesman said: "The issue has moved on significantly and progressed since last summer, with the groundbreaking and historic concordat signed by the Scottish Government and COSLA last November to deliver year-on-year progress in reducing class sizes."