Mr Wilson was responding to a TES Scotland survey of all 32 councils which revealed for the first time the extent of the problem in Scotland. The findings show:

- One in six P1-P3 classes in more than a fifth of primary schools have 31 or more pupils.

- Almost one in six pupils is affected.

- Extra classes would require an additional 258 teachers and 180 classrooms in the 24 councils that were able to provide estimates.

There is a wide variation across the country, however. The Government’s policy poses no problems for Argyll and the three island councils. But Dundee, East Renfrewshire, Edinburgh and North Ayrshire (which covers Mr Wilson’s Cunninghame North constituency) say around a third of primary 1 to primary 3 classes have more than 31 pupils. Half of the primaries in Dundee, East Dunbartonshire, East Renfrewshire and Edinburgh would be affected.

Although the reduction in class sizes is to be achieved by the six-year phasing out of the #163;14 million assisted places scheme, councils have become increasingly alarmed at the significant costs they would incur and the upheaval schools could face for the sake of cutting classes by two or three pupils.

The extra teachers, for example, could cost almost #163;6 million. The 180 new classrooms would add more than #163;13 million in capital costs. And many primaries simply do not have the room to expand, particularly schools that are popular with parents where the pressure on accommodation is most acute.

John Travers, president of the Association of Directors of Education in Scotland, said in a special statement: “It is clear that the position is complex and that each case has to be considered individually. A key issue is the question of placing requests. If education authorities were able to limit class sizes to 30 by refusing placing requests, this would go a long way to addressing the problem. In one authority [Renfrewshire], 16 new classrooms would be needed if placing requests up to 33 continue, but none would be necessary if a limit of 30 was imposed.

“A similar picture emerges in relation to additional staff. Using two teachers with a class of 33 is both wasteful of resources and extremely costly.”

Councils will now press Mr Wilson to discover how far his flexibility extends. They want scope to turn away placing requests, which ll require legislation and prove controversial among the middle-class parents being wooed by New Labour. They also believe that creating larger composite classes would avoid parental anger and achieve class maxima of 30 without extra expenditure.

But composite classes are fixed at a statutory limit of 25 and any change requires a deal between unions and management in the Scottish Joint Negotiating Committee, whose agreements are legally binding. The Educational Institute of Scotland would be very reluctant to support larger composite classes to pay for other improvements, Ken Wimbor, the unions negotiating secretary, said.

Keir Bloomer, director of education in Clackmannan, believes the best way to raise educational standards in the early primary years would be to give councils the resources to employ additional nursery nurses and learning support staff. This approach is widely supported in the directorate and would be in line with the Governments early intervention strategy.

A lower contractual class maximum is a very rigid device and, given the small reduction from the existing maximum of 33, will make very little impact in educational terms, Mr Bloomer said. Limiting the size of early years classes would generally lead to smaller classes from primary 4 upwards so resources would not be targeted towards the early years.