Some schools in England may be breaking the law in the way they admit pupils, despite the introduction of new rules last year, the government has said.
Schools are now making arrangements for 2009 entry
Schools Minister Jim Knight said not all children had been given an equal chance of getting into a school of their choice this September.
Councils who failed to prevent this have been warned they may have flouted the new admissions code themselves.
Mr Knight said "covert admission practices" penalised poorer families.
The independent Office of the School Adjudicator, which regulates the admission system, investigated objections against 79 schools last year where admission criteria and practices had breached legislation or the code.
Some heads had asked to see parents' marriage certificates, while others invited parents to an interview - a practice banned under the new code.
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The Department for Children, Schools and Families said other examples of law-breaking by admission authorities included not giving the highest priority to children in care and even asking the order in which parents had ranked their school choices.
Speaking on BBC Breakfast, Mr Knight said most schools complied with regulations, but some did not.
"It might be asking to see parents' marriage or birth certificates, it might be as simple as interviewing them - which is a completely subjective and unfair measure," he said.
"It might be requiring a school uniform which is prohibitively expensive. All of these things we rule out in law because we want to make sure that every parent has an equal chance of getting into popular schools."
Rosemary Sanderson is a parent whose son was refused a place at a Roman Catholic comprehensive.
She told BBC Radio 4's Today programme the appeals process had been "humiliating".
"It's stressful, it's unjust, I felt ashamed at the appeal process - to be competing with other parents, all of whom had children who were deserving of a place at a good school."
Mrs Sanderson said she found that the parents who "begged and pleaded the most" and were the most articulate got the place for their child.
Mr Knight said he hoped the new code would help end problems suffered by parents like Mrs Sanderson.
The admissions code was toughened up in an effort to placate Labour backbenchers rebelling over the government's introduction of trust schools, in the Education and Inspections Act.
The crucial change was that admission authorities must now "comply with" the code, not merely "have regard" to it, as had been the case.
It ruled out subjective arrangements which penalise low-income families or vulnerable children with disabilities, special educational needs or in care.
In his annual report last year, admissions watchdog Dr Philip Hunter said strategies that could upset "articulate parents" were needed to make school admissions fairer.
He said sought-after schools could "cream off" children in neighbouring areas, leaving some schools there with too many children from deprived homes.
In a letter going to all local authorities, Mr Knight said: "I am very concerned that formal complaints and other anecdotal evidence suggest that some local authorities and schools are not complying with the law.
"No ifs or buts - there is absolutely no excuse not to comply with the law to stamp out unfair and covert admission practices, which penalise low-income families and increase social segregation.
"It is unacceptable that children may be missing out on school places because unlawful arrangements are in place almost 12 months on from the code becoming statutory."
Bans assessments of parents' financial, marital or social status
Bans schools from favouring those who put them first
Bans the use of subjective tests and criteria eg. asking why parents want that school
Bans giving places based on children's interests, knowledge and hobbies
Allows schools to give priority to siblings
Gives highest priority to children in care
Says arrangements must be fair, objective and clear
He sent the letter as a new admission appeals code came into force. The code sets out to balance the right of parents to a fair hearing and the right of schools to some protection against having to admit more children than originally planned.
Mr Knight said: "Local authorities and schools must be able to set whatever admission policies they consider fairest for their particular area, adjusting for local conditions.
"But it's vital that parents who appeal have full confidence in the system and that local authorities and schools do not leave themselves open to challenge."
A spokesman for the Local Government Association, which represents councils in England, said: "The letter from the minister will be given careful consideration by all local authorities.
"Councils strive to make certain that parents and children have a fair chance to get into a school of their choice and local authorities will do everything they can to ensure that is the case."