A new code on admissions came earlier this year
Half of England's schools which control their own admissions have been breaking the law, a survey suggests.
Checks were carried out for the Schools Adjudicator, which regulates the admissions system in England.
Of 3,500 faith schools, foundation schools and academies checked, half were breaking a new admissions code.
Chief adjudicator Sir Philip Hunter said many schools were asking unfair questions on application forms such as parents' occupation.
The new code was brought in earlier this year to make the admissions system fairer and more transparent.
Sir Philip was asked by the government to compile a report on school admissions after it found examples of a handful of faith schools breaching the code, for example by asking for donations or to see copies of parents' marriage certificates.
He revealed his interim findings at a conference on admissions in London, as tens of thousands of parents in England apply for secondary school places.
"The secretary of state correctly identified a series of issues about compliance of the admissions code. He asked me to sort it out," said Sir Philip.
He was "pretty confident" this had been done.
A survey of all local authorities' policies had been carried out, as well as checks on about 3,500 schools which control their own admissions.
Local authorities control admissions for the majority of England's 21,000 state schools - but many, including faith schools, grammar schools, foundation schools and independent academies - set their own policies.
These policies are meant to be in line with the new admissions code. Sir Philip said most of the breaches of the code uncovered in the survey were administrative, rather than intentional on the part of the schools.
"Actually what most of them were about was a whole lot of problems about definitions such as schools failing to define distance," he said.
"There were also issues about supplementary information forms and schools asking things they shouldn't ask."
He said the schools had not used the answers, but that they had been asking for information they were not allowed to - such as the occupation of a candidate's parents.
"Simply nobody was realising that supplementary forms were part of the admissions procedure, but now they know," he added.
Schools and education authorities had already been informed about the problems and Sir Philip said he was confident that most would have been addressed in time for this year's applications process.
His full report will be published next month.
Liberal Democrat schools spokesman David Laws said the adjudicator's findings were "shocking."
He said: "Cases where schools are asking questions about a pupil's background are totally unacceptable.
"It is parents who should be choosing schools and not schools cherry-picking children.
"Any administrative problems need to be urgently resolved so that parents know exactly where they stand when choosing schools for next year."
Sir Philip was speaking at a conference attended by council and education officers involved in school admissions, organised by a think-tank which is part of the Local Government Information Unit, which advises local authorities.
The body - the Children's Services Network (CSN) - told delegates more schools should use lotteries or random allocation to decide who to admit.
And it said restrictions should be placed on schools regarding the awarding of places to pupils who have brothers or sisters at a school.
For example, if a family moved out of a catchment area once their eldest child had been given a place at a school, their other children should not be admitted automatically.
And more use, it says, should be made of "fair banding". This is where prospective pupils sit tests and are then grouped into bands - and a school then admits a proportion from each group.
Local authorities have a statutory duty to promote fair access to education.
Martin Rogers, CSN policy consultant, said: "In many areas, there is a significant degree of segregation in school intakes, arising largely from admissions criteria that give priority to siblings of children already at the school and to children who live in close proximity to the school.
"We hope admission authorities will more actively consider the use of alternatives such as banding or random allocation to improve the position."
The Catholic Education Service for England and Wales said most of the breaches were of a minor and technical nature which it was working to correct.
"Sir Philip has acknowledged to us the hard work and considerable good will that has been demonstrated by all involved in the process as they work together to ensure a fair admissions process for all," said chief executive Oona Stannard.
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