The leader of Kent County Council warns that grammar schools are reducing their entry standards in order to fill places.
Paul Carter believes schools are being motivated to fill their classrooms by financial incentives.
And the Conservative says that lower academic standards will undermine the specialist purpose of grammar schools and will affect non-selective schools in the same catchment area.
His comments come as thousands of children return to school across Kent this week. The county has 32 grammar schools, including three in the Canterbury district – Barton Court and the Boys’ and Girls’ Langtons.
Grammars are supposed to cater for the top 25% academically, but that figure is now at 31.8% in Kent.
Mr Carter believes this is partly because of the schools which have become academies and control their own admission procedures.
“Many now set their own pass rate and will fill the school up no matter what,” he said. “If you were a governor of a grammar school and every pupil that comes along is worth nearly £5,000 you want to try and fill the grammar school up and have full forms of entry.
“The tendency now is to set a pass rate that fills the grammar school. I think you have got to be careful that you don’t dilute the specialism of grammar schools, which are there to provide a learning environment for the highly academic pupils.”
Peter Read, a former headteacher and commentator on education in Kent, believes the pattern is unlikely to change.
He said: “While showing unhappiness about the situation, Mr Carter identifies his own reasons for the increased proportion.
“But he gives no indication there is an appetite to wind back the proportion of children going on to grammar school.
“Indeed, I don’t believe that with the loss of control by KCC to individual academies this would be possible.”