The detailed study, carried out by Coopers and Lybrand for the Local Government Association, shows more than half of all classes with 30-plus pupils result from admissions and appeals problems.
The authors say education secretary David Blunkett must implement five major solutions, including team teaching, using other schools with undersized classes and revising standard admissions to multiples of 30, if Tony Blair's promise to cut class sizes for all 5,6 and 7 year olds is to be kept.
LGA education chairman Graham Lane welcomed the report and called on Blunkett to take the advice on board.
`There is sufficient money in the system, but ministers need to address the problem of mixed age schooling, admission appeals and cut the bureaucracy surrounding entry.'
Lane met education minister Stephen Byers yesterday to discuss the report and other areas of schooling, including Local Management of Schools and teachers' pay and conditions.
He said: `We commissioned this study because we are keen to see the pledge carried out for the sake of our children and our schools. Smaller class sizes are fundamental to better teaching.
`The government must realise that pumping money into the system alone will not do nor will edicts from Whitehall and civil service mandarins.'