Some academy schools may be "manipulating" admissions to select and exclude certain pupils, a new report suggests.
A study by the Academies Commission says there are concerns that the rise in schools taking on academy status could fuel social segregation, rather than reduce it.
It warns that some academies may be "covertly" selecting pupils by using extra information on families when making admissions decisions, or holding social events with prospective parents.
The concerns are raised in a new report by the independent Academies Commission, which is examining the impact on standards of more schools becoming academies.
The Commission said it had received evidence and research that some popular schools, including academies, are attempting to select and exclude pupils.
It says that this practice is not new, but the fact that academies have autonomy over their admissions has "attracted controversy and fuelled concerns that the growth of academies may entrench rather than mitigate social inequalities".
The Commission said it had heard examples of some academies "willing to take a 'low road' approach to school improvement by manipulating admissions rather than by exercising strong leadership".
Under the current system, all state schools must abide by an admissions code, which says they must admit pupils in a fair and reasonable way. But the Commission's report said it had received numerous submissions suggesting that "academies are finding methods to select covertly".
Schools and academies can ask prospective families to fill in a supplementary information form (SIF) when making an application for a place. The report says that research shows that some schools, particularly those in charge of their own admissions, were asking for parents to fill in lengthy forms, involving open questions, and sometimes asking for information not allowed under the admissions code.
"Such practices can enable schools to select pupils from more privileged families where parents have the requisite cultural capital to complete the SIF in ways that will increase their child's chances," the report said.