Every local education authority must draw up immediate action plans outlining how they intend to reduce class sizes for five to seven-year-olds.
No deadline for the plans has been set as yet. However, they will be subject to approval by Education and Employment Secretary David Blunkett.
Although popular with many parents and teachers, the measure faces opposition from those who believe restricting class sizes will limit families' choice. Some parents in the London borough of Richmond, for example, support infant classes of more than 30 because they give them more chance of getting into the school they want.
Labour's promise on class sizes must also counteract the rising national pupil:teacher ratio which last month reached a new five-year high.
The feasibility of its plans have been widely questioned, although they were supported by analysis from the National Foundation for Educational Research.