Schools were accused of breaking the admissions code via backdoor selection processes in a report released this week.
The study, by the London School of Economics' education research group, stated that hundreds of schools are asking parents for additional information about themselves and their children that will enable them to select the best pupils.
About 500 schools use supplementary information forms, with some asking questions about the parents' marital status and another even asking for a 100-word personal statement from the hopeful pupil.
The study also found that there had been an increase in selection by ability or aptitude from three per cent in 2001 to five per cent today.
Among the findings was the practice of staging "meetings" in schools. Interviewing parents and their children is in direct breach of the admissions code introduced in 2007. However, some schools set up the meetings to "discuss the applications".
LSE's Professor Anne West, the report's author, said that a "totally independent body" should be set to oversee the admissions process.
She said: "The system needs to be changed if admissions are going to be fair.
"You could have all the applications coming into the local authority or to a church body. On the basis of chosen criteria, someone else could then make the decision about whether a child is accepted or not.
"What you need to have is the criteria set up so that another outside body could make all the decisions.
"Then there is no scope for discretion, and you can't choose a pupil based on whether somebody falls into a particular category that you're looking for."