School inspectors revealed yesterday that the cost of cutting the average primary school class by three would be pounds 540m and said there was no evidence to justify the expense.
A report by the Office for Standards in Education (Ofsted), the independent standards watchdog, also challenged Labour's costing of its pledge to limit class sizes to less than 30 for children aged under seven.
Inspectors said that reducing average primary class sizes by one would cost pounds 170m and by two pounds 350m. Labour's promise to cut class sizes would cost not pounds 60m, as it had suggested, but at least pounds 180m.
Research in Tennessee in the United States showed dramatic improvements in five-year-olds' reading and arithmetic when they were taught in classes of 15, but that would cost pounds 1.2bn.
The inspectors' report, based on evidence from visits to 594 secondary and 1,173 primary schools and covering 200,000 lessons, found no clear link between class size and standards.
But it did show that children aged five to seven did better in smaller classes. It challenges the present arrangements under which secondary schools are better funded than primaries.
Teaching quality, the inspectors said, was much more important in determining standards than class size. They proposed spending more on classroom assistants rather than on reducing class size.
They would be cheaper than trained teachers - or some would be volunteers. They would help with reading, music and with difficult children
"Reducing class size across the board by even a small amount is expensive; there is no evidence to justify this."
Teachers, who have been campaigning with parents for more public spending to stop class sizes rising, accused the inspectors of telling the Government what it wanted to hear.
Chris Woodhead, Her Majesty's Chief Inspector of Schools, said his office was "neither the poodle of Government nor the champion of the teaching profession".
He said: "Heads and governors need to think about how best to use their resources to maximum effect. They ought to be asking whether reducing classes by one or two would have that effect or whether they could do better by appointing classroom assistants."
The report found that standards were higher in large teaching groups of 36 or more than they were in classes of 28. The reason, it suggests, may be that many of these classes had two adults in them, a teacher and a classroom assistant.
Four per cent of classes had 36 or more pupils but in half of these there were good reasons such as bringing children together for singing or PE.
David Blunkett, Labour's education spokesman, said: "The inspectors have confirmed Labour's belief that a reduction in class sizes for the early years of primary school together with improvements in teaching are key to raising standards."
David Hart, general secretary of the National Association of Head Teachers, said: "Ofsted have entered the political arena by falsely stating that there is no evidence to justify the investment of money in reducing class sizes."
The independent National Foundation for Educational Research said its costing of Labour's proposal, based on targeting resources in a local authority, was correct. But Christine Agambar, of Ofsted, said the figures did not take into account that young children could not travel large distances and more places would be needed at their local school.Reuse content