Schools which reserve places for children who make them their first choice risk having their admissions policy overturned because it is unfair to parents.
In a series of landmark rulings, schools adjudicators have ordered schools in Kent and Calderdale to scrap the so-called "first preference first" oversubscription criterion.
They found it distorted the admissions system, forcing parents into tactical choices rather than enabling them to list schools in their true order of preference.
The adjudicators, Dr Philip Hunter and Alan Parker, said the use of the criterion by six Kent grammar schools forced parents to make a difficult choice between them and grammars in neighbouring Bromley, which select using only 11-plus results.
"They can gamble on securing a place at a Bromley grammar and accept a non-selective school if their child does not reach the required standard; or forgo that option for a better chance of securing a place at one of the north-west Kent grammar schools," they said.
In a separate report, Dr Alan Billings, another adjudicator, made similar conclusions after grammars in Calderdale complained the same system was being unfairly employed by the local council in admissions policies for community schools. The decisions come despite "first preference first" being specifically approved in the Government's admissions code of practice.
The code is being tightened up after a critical report by the Commons education select committee said current admissions policies "eroded the role of parental preference".
The Government is pressing ahead with plans to increase the number of schools which gain foundation status and become their own admissions authority, despite criticisms that this will make the admissions system more confusing for parents.
A consultation paper on allowing primaries to make fast-track applications to become foundation schools was published earlier this month.
Secondaries will be able to use the fast-track procedure from August.