England's chief schools adjudicator has warned that thinning down the admissions code for allocating school places fairly could risk weakening it.
The government wants a simpler code, but has not given details.
Ian Craig said the current code could be made more easy to understand, but cutting it down risked "throwing the baby out with the bathwater".
He also warned that some faith schools were inadvertently using criteria that benefited white middle-class families.
Mr Craig said the code, which contains both legal requirements and good practice, had been improved over the years.
Removing the non-binding guidance would risk "throwing the baby out with the bathwater", he warned.
"I don't think it should necessarily be cut down in terms of its requirements, but it needs to be more accessible," he said.
"I think we need to be very careful that while we're making it more accessible we don't simplify it to such an extent where it becomes a useless document," he said.
The annual report of the schools adjudicator, published on Monday, said that 18 of the 152 local authorities in England said some of their schools were not complying with the code.
Mr Craig's report also raised concerns about the criteria used by faith schools to determine whether children and their parents were members of a particular denomination.
He said some schools and diocesan authorities were using points systems relating to "involvement in activities that are beyond those that could reasonably be expected as part of religious membership or practice".
For example, it would be unfair to discriminate against the child of a working mother who was unable to help clean the church hall because of her employment hours, he said, or to expect parents to have had their children baptised in the first six months when this was not the cultural norm among Eastern Europeans of a given denomination.
"We have faith schools that benefit the white, middle-class areas but don't benefit the immigrant population," he said.
He added: "I don't generally think we've come across schools that have done that to skew their intake specifically, but our view is it has been skewing the intake."
A Church of England spokesperson said: "All Church of England schools comply with the Government admissions code and most are inclusive, serving children of all faiths and none."
The Church pointed out that 40% of its schools are in rural areas and "by and large they serve their immediate community".
It said some of its schools in urban areas "are over-subscribed and apply discriminating criteria which may include a church or Christian family background".
Alison Ryan, education policy adviser at the Association of Teachers and Lecturers (ATL), said: "While many state faith schools are excellent schools, with mixed pupil intakes, some voluntary aided schools, whether intentionally or not, are less diverse ethnically and have fewer disadvantaged pupils because of the faith criteria they use in their admissions policies.
"We urge the religious authorities involved to condemn this effect and open up their schools to the wider community," she added.
'Unclear or invalid'
A small number of schools carry out tests for aptitude in a particular subject, such as music and sport, but are not - apart from existing grammar schools - supposed to test on the basis of ability.
But Mr Craig said it was very difficult to distinguish between the two.
"It is very difficult to define the difference, even if you're looking it up in a dictionary," he said.
His report said some schools' testing methods "were often unclear or appeared to be invalid".
The admissions code was tightened under Labour, and this year was the first full year during which the revised code was applied.
Parents' appeals over school places are heard initially by panels at local level, but can be referred on to the schools adjudicator.
The report said the number of referrals from parents and local authorities had increased this year, from 201 last year to 387 this year.
Education Secretary Michael Gove said he intended "to make the school admissions framework, including the School Admissions Code, simpler and fairer".
"I am committed to driving up educational standards so all parents have that choice of high-quality schools close to home, which is why we are encouraging providers to set up new schools and turning round under-performing schools. And so no child is disadvantaged because of their background I am introducing the pupil premium," he said.
The government says it has allocated £2.5bn a year, by 2014, to be allocated to schools on the basis of how many disadvantaged pupils they take.
Not all of the money is new to the schools budget however, and the policy may mean funding cuts for schools with fewer poor pupils.
The government is understood to be considering allowing schools to prioritise children from disadvantaged backgrounds in their admissions policies.