Fiona Hyslop told the annual conference of the Association of Headteachers and Deputes in Scotland last week that she would not insist on limiting all P1-3 classes strictly to 18 pupils - a move that will be seized on by her political opponents who will argue that the SNP had not thought through one of its key manifesto pledges.

Meanwhile, The TESS has been shown the report of the working group set up by former education minister Peter Peacock into class size reduction, which is likely to offer no clear recommendations on policy. It says previous research is inconclusive.

In a week when ministers came under sustained parliamentary pressure to spell out the extent of the Government’s commitment, Ms Hyslop told AHDS members: “I recognise the need for a flexible approach to this. I want to be reasonable. If there is a class of 19, I won’t go to the wall on it. I trust headteachers to manage it effectively.”

The minister recognised that schools had different layouts and that the issue had to be approached intelligently. “I don’t want to see a P1 class of 25 disrupted just so it can be reduced to 18,” she said.

However, when asked whether classes in the upper primary would also be reduced to 18, Ms Hyslop said she could not guarantee such a move in the four years of the current parliament.

The early years were her priority, she insisted, and added: “We have to start somewhere.”

Although critics may make political hay, Ms Hyslop’s decision echoes that by her Labour predecessors who allowed secondary heads to vary S1-2 class size limits.

The issue sparked some of the most heated exchanges of the new parliament last week. Wendy Alexander, the Labour leader, accused Alex Salmond, the First Minister, of “a betrayal of the parents and teachers throughout Scotland who believed those promises”, when he failed to give precise details of costings and timescale.

Mr Salmond accused Labour of reneging on its promises, quoting back at Ms Alexander her words: “Class sizes are not a good measure of what matters.”

Rhona Brankin, Labour’s schools spokesperson, cited figures obtained from local authorities to undermine the SNP. Glasgow would need an extra 397 and another 186 classrooms at a total cost of pound;47 million just to limit P1-P3 class sizes to 18; North Lanarkshire would have to hire 150 extra teachers and build 83 new classrooms at a total cost of pound;20.5m; Edinburgh’s estimate was 206 additional teachers, with a total cost in staffing and capital spending of pound;42 million.

The political spats drew fire from the Educational Institute of Scotland, which attacked all three opposition parties for promising class size cuts in their election manifestos and now trying to wriggle out of their promises.

Ronnie Smith, EIS general secretary, said the only division between the Scottish National Party, Labour and the Liberal Democrats had been about the depth of cuts in class sizes and the focal point for reductions.

“You would have thought among the three of them, they might have been able to find some common ground. If they can’t work something out together on this one, where the direction of travel for all three was the same, then it does not give you many grounds for optimism,” he told The TESS.

Politicians were claiming a choice had to be made between having smaller class sizes or having good teachers, he said, but both should be possible.

John Stodter, general secretary of the Association of Directors of Education in Scotland, offered another alternative: focus cuts not on the early years but on P6-S2, when attainment flattens out and there is a lack of progression. He called for an end to the “roller-coaster” of class sizes from nursery to higher education: nursery - 25; P1-3 - 18; P4-7 - 33; S12 maths and English - 20; practical classes - 25; other classes - more than 30; S6 - 5 or 6 in many classes; and finally, university - up to 300.

Report p4.