Grammars are advised against becoming academies
Grammar schools in England are being advised not to apply for academy status.
The National Grammar Schools Association has issued an "urgent statement", warning governors and head teachers of "covert dangers".
All state schools in England have been invited by Education Secretary Michael Gove to become academies free from local authority control.
Ministers said academy grammars would still be able to select on ability.
In a statement, the NGSA said: "We strongly advise governors and head teachers to be extremely cautious.
"At present, there are fears that academies may not be legally defined as 'maintained' schools, in which case they may lose the statutory protection of requiring a parental ballot before they are turned into comprehensive schools.
"There may be other covert dangers and until everything is made clear in the area of legislation and elsewhere, we strongly recommend extreme caution."
NGSA chairman, Robert McCartney QC, said: "The problem is, if a grammar school decides to become an academy what is the protection for the parents? Does there have to be a ballot, does it become an all ability intake?
"All academies to date have been all-ability intakes, there doesn't seem to be any provision for safeguards in order to turn grammars into academies."
"Since this new freedom from local authority control will mean that schools have control over their own admissions procedures, would it be possible for a new small group of people, the trust and governors, to decide that admissions procedures are going to change and that anyone who wants to be in the school can come into the school?"
Mr McCartney suggested the academies proposals had arisen from the Conservatives' desire not to be labelled as "elitist".
The Tories distanced themselves from supporters of grammar schools under David Cameron's leadership.
A spokesman for the Department for Education said: "We want grammar schools to gain academy freedoms if they want them, as we do with all schools.
"Head teachers know how to run their schools better than bureaucrats or politicians, and it is for them and their governors to decide whether to convert to academy status.
"Where grammar schools exist we support them. The Academies Bill will not alter the selection arrangements for grammar schools if they choose to gain academy freedoms."
Grammar schools are state schools which select on the basis of academic ability, demonstrated in the 11-plus exam.
There are 164 grammar schools in England, many concentrated in areas such as Buckinghamshire and Kent.
There are no grammar schools in Wales and Scotland
In Northern Ireland the 11-plus has been abolished, but the Executive has not reached a consensus on the way forward.