The Court of Appeal gave the go-ahead for the review last week.
A spokesman said the borough had urged the county council to withdraw the statutory notice to set up the school and stop wasting money.
The Pounds 12 million school for 1,000 pupils in Campbell Park would be the first grammar to be built in Britain for 30 years.
Milton Keynes, which is due to become a unitary authority in 1997, is the only part of Buckinghamshire which does not have a grammar system.
About 400 of the 8,000 secondary-aged children travel to Aylesbury to attend selective schools.
Labour and Liberal Democrat borough councillors were angry that county councillors failed to take notice of residents' opposition to the plan which they voted in by 35 votes to 29 last May.
"The people of Milton Keynes said three times they do not want a grammar school," said the spokesman.
As The TES revealed earlier this month, a former Conservative councillor, now sitting as an independent, wanted the grammar school application to be withdrawn.
Peter Mullins said that he was opposed to the county council - in its dying days - seeking to inflict a grammar school on another authority which should be allowed to decide such issues for itself.
A date for the judicial review has yet to be decided.