The Pounds 12 million school in Campbell Park, Milton Keynes, would be the first grammar to be built in Britain in 30 years if approved by the Education Secretary, Gillian Shephard.
But borough councillors will be applying to the High Court for a judicial review to prevent the school being built because they claim the county has failed to heed the opposition of residents to the proposals.
Milton Keynes, which is to become a unitary authority in 1997, is currently the only part of Buckinghamshire which does not have a grammar system. Around 400 of the city's 8,000 secondary-aged children travel to Aylesbury to attend selective schools.
Labour and Liberal Democrat councillors are angry that the county has ignored popular feeling in the area and plans to force a grammar school on a council which does not want it. County councillors voted by 35 votes to 29 in favour of the scheme.
The city's teachers are also opposed to the scheme on educational and financial grounds.
If the project goes ahead the school is scheduled to open in 1998 in the Campbell Park area of the city. Councillors were due to issue public notices to establish the school as The TES went to press.
Nigel Long, the borough council leader, said: "The council is taking this action because it believes the consultation process carried out by the county council was flawed. The process by which it took the decision was wrong and not carried out properly. The county council did not take regard of the opposition by Milton Keynes residents."
Last year two consultations with residents of Milton Keynes revealed a majority against the idea of a grammar school. But the second survey found 60 per cent of parents with children aged eight or under were in favour of the grammar school.
Crispin Graves, Buckinghamshire's education committee chairman, was unavailable for comment.
But Andy Dransfield, a Tory county councillor representing Milton Keynes, said: "Any attempt to take this matter to the High Court would be a waste of money. The present borough council is not the unitary authority, and there is no saying how the new authority would look upon this matter.
"The comprehensive lobby is claiming a grammar would destroy the existing system, but the cream-off would be minimal. We cannot see why parents in Milton Keynes should have to transport their children to grammar school in Aylesbury at their own expense when they could have a school on their doorstep."
The grammar school is one of three secondaries being planned for opening in Milton Keynes before the end of the decade.