They are calling on the Government to change the ballot system because thousands of parents who live near grammars will be denied a vote.
"Our worst fears are coming true, said Margaret Tulloch of the Campaign for State Education.
"We've written to (education minister) Estelle Morris to ask why it is that parents who pay their council tax to maintain these grammar schools can't get a say when people who live miles away will do?" In Barnet, north-east London, which has three grammar schools, only a third of the authority's 91 primary schools are eligible to vote. Another 38 feeder schools can vote, most of them outside Barnet. Fourteen of these are private schools.
In Ripon, North Yorkshire, 11 out of 16 local infant and junior schools get a say, although another eight schools outside the area are also eligble.
Under the current rules, two types of vote will decide the future of selective education. In areas where 25 per cent or more of all school places are selective - such as Kent and Medway - all local parents get a vote.
However, in areas where there are fewer grammar schools, only schools which have sent three or more pupils to a grammar school in the past three years can vote.
At least 20 per cent of eligible parents must sign a petition to trigger a ballot. But anti-selection campaigners fear most parents in schools outside the borough will not bother. Barnet campaigner Jenny Brown said: "The handful from the outside schools who send their kids to Barnet grammars want the grammar schools to stay. The rest won't give a damn."
Campaigners have until July to gather petitions if they want a ballot later this year. However, most local campaigners plan to wait until the autumn, delaying a ballot until 2000.