They say that the concentration on reducing class sizes for five to seven-year-olds will see more pressure on resources for children higher up the primary age range.

The findings come in a survey of more than 30 education authorities by the National Union of Teachers.

Doug McAvoy: "Inevitable"

The authorities also say there is more chance of children being bussed to schools outside their locality as local schools reach the government's class size limit.

Reducing class sizes for five, six and seven-year-olds was a key manifesto pledge for Labour at the last General Election.

Hundreds of millions of pounds are being spent on employing new teachers and building extra classrooms.

Last autumn, ministers announced that 140,000 infant school children were starting the new school year in smaller classes.

The General Secretary of the National Union of Teachers, Doug McAvoy, said: "It was inevitable that the speed with which the government moved to limit class sizes would create problems, but that should not detract from the benefits the reform will bring to the education of children now in smaller classes.

"But knowing the problems allows both local education authorities and the government an opportunity to overcome them or limit their impact."