The study by London University academics suggested that 12-year-olds in the first year of secondary school could also potentially benefit from small class sizes.

But despite evidence that smaller groups yield results, there has been an increase in "whole class" teaching, which was traditionally associated with children learning by "rote", the report said.

The findings followed a row over suggestions by Schools Minister Jim Knight that classes of up to 70 were "perfectly acceptable".

The research was conducted as part of the Cambridge-based Primary Review, the biggest inquiry into primary education in England for 40 years.

Academics at the Institute of Education and King's College London examined the impact of different ways of organising pupils into teaching groups.

The study said: "The age of the pupil is important when considering class size effects.

"There is a clear case for small class sizes in the reception year, but research also shows where resources could be further targeted.

"That is, on achieving classes smaller than 25 for those children with most ground to make up in literacy. Another important implication is to maintain smaller classes from one year to the next where possible."

They said schools should consider grouping children within their classes and allowing them to interact with each other more in lessons.