Ministers are prepared to sacrifice parental choice, the appeals system and the right of schools to spend their budgets how they wish in order to meet the manifesto pledge - even where parents prefer bigger classes.
A class of 29 children with one teacher is considered a "good thing", a class of 31 with extra support would be unacceptable, officials currently visiting local authorities have revealed.
The move is certain to anger parents who have reached local agreements with headteachers and council officials. Many are more than willing for their children to be in large classes in good schools.
John Fowler, assistant head of education at the Local Government Association, said: "There is a fine balance between reducing class size and restricting parental choice."
More than half of primary pupils are taught in classes of 31-plus. There are more than 1.1 million in classes of 31 to 35, another 120,354 are in classes of 36 to 40, and almost 9,500 in classes of 41-plus.
Class sizes have been rising steadily in primary schools despite attempts by LEAs to pump money into them because local management of schools has allowed heads and governors to set their own spending priorities. Large classes have enabled heads to spend more on classroom support and teachers.One school with more than 30 children in each of its six key stage 1 classes has been able to fund another 4.9 teachers' jobs because of the extra cash the pupils bring in.
Civil servants charged with turning the class size promise into reality by 20012002 are now looking at authorities with some of the best and worst records on class size.
A five-strong team from the Department for Education and Employment is visiting 11 local authorities to discuss how to implement the pledge. Ironically its leader, civil servant Neil Remsbery, is understood to have worked for the previous government on reasons why it was not possible.
The authorities range from Kingston-upon-Thames, where seven out of 10 children are in classes of more than 30, to Islington, where 93 per cent of pupils are in classes of 30 or fewer.
Officials stressed the choice of LEAs was based on a statisticians' view of what would offer a representative cross section, with differing proportions of large classes, surplus places and anticipated demographic changes.
Kingston, however, one of the top 10 performing authorities in this year's tests for 11-year-olds, will be the acid test of the Government's commitment.
Schools in the borough draw in children from Surrey, Sutton, Merton, Wandsworth and Richmond, and there is no spare space in its primaries.
Education director John Braithwaite estimated the council would need another 45 classes to fulfil the pledge - and that comes at a price both in buildings and teachers.
Kingston's call for more cash is already being echoed throughout the country. As council officials estimate that a middle-of-the-range classroom costs #163;50,000, the DFEE will face a multi-million-pound demand.
Ministers, however, insist that no money other than that saved from the abolition of the Assisted Places Scheme - #163;200 million - will be used to meet the pledge.
They have made no decisions yet on how the money will be allocated. An announcement is due in the autumn.
But they have signalled their intention to sacrifice parental choice in a consultation paper on the White Paper Excellence in Schools.
"Reducing class size may mean limiting the number of places available for admission to some primary and infant schools, " they admitted in the paper,which foreshadows the Education Bill.
"The legislation will also need to preclude admission appeal committee decisions which would result in such classes of more than 30 pupils."