Two-thirds of schools that control their own admissions have failed to comply with rules for it, according to a study for the Office of the Schools Adjudicator.
A team of barristers who analysed a sample of 3,000 schools also found that half of local authorities, which are responsible for admissions at the remaining state schools, had also broken the code, which came into force in 2007.
However, Sir Philip Hunter, the chief schools adjudicator, said that the breaches "derived from misunderstandings of the code, not from wilful disregard to it." Among the 3,000 cases examined were 2,244 breaches relating to the definition of terms such as "sibling and distance".
Another 803 breaches involved schools asking for supplementary information, such as parents' places of work, which might be used for back-door selection.
Ed Balls, the Schools Secretary, said: "These findings support the action we are taking to strengthen the system to ensure compliance with the law."
Admissions analysis, pages 26-27.