The most compelling evidence in support of the move comes from the STAR (Student Teacher Achievement Ratio) project, a survey of 7,000 pupils in Tennessee schools, which found that those in small classes did best, especially in mathematics and reading.
Peter Mortimore, director of London University's Institute of Education, has advocated smaller class sizes for younger children and has called for similar studies to be set up in this country.
The National Commission on Education has declared that within five years no primary class should contain more than 30 children. At present, around a quarter exceed that figure.
In Scotland, a class size of 33 is tied to teachers' conditions of service. In England and Wales there has been no legal limit since the provisions of the 1944 Education Act which prescribed 40 pupils in primary and 30 in secondary classes were revoked in the 1950s.
Ivor Widdison, administrator of the Council for Local Education Authorities, believes that schools should be given the choice between smaller classes and more non-contact time for teachers.
"We greet this as good news. But it can only be a long-term aim. We need to lighten the teaching load of new teachers and you can't do that at the same time as having a firm agreement on class sizes. Most LEAs would like to have the choice.
"Parents don't believe this talk about class sizes not having any bearing on achievement. It's common sense that it makes it easier for teachers to do their job."
Alan Parker, education officer of the Association of Metropolitan Authorities said it was a politically shrewd move. "He has obviously taken good advice - he's responding to a strongly felt desire on the side of parents to see their children taught in smaller classes."
But making the limit legally enforceable could cause problems: "Once you start to legislate you run into practical difficulties. What do you do if you have one more kid than you should have - make them go and sit in another room?" Providing the money to reduce teacher:pupil ratios would have the knock-on effect of improving working conditions and stopping teachers leaving the profession. "Large class size is one of the things that drive people to work elsewhere."
Sue Nicholson, assistant secretary of the National Association of Head Teachers, warned that targeting lower age groups could displace the problem and any move to reduce classroom numbers would require a thorough review of the funding system under local management of schools. "Unless they put their money where their mouth is then it might mean that the size of groups goes up as the children get older."
Legal enforcement could present small schools with particular problems, she argues, a point taken up by David Whitehead of the Association of County Councils.
"Say you imagine a primary school with a one-form intake of about 30. It might never be exactly 30 but that might suit the school and the parents and children. If you have a rule about maximum class size, there could be difficulties."
However, Mr Whitehead believes the policy pledge would have the overwhelming sympathy of the education system.