Thousands are enrolling young children in fee-paying preparatory schools - where they enjoy smaller classes and more teacher attention - to maximise their chance of getting a free grammar school place at 11.
Around 15 per cent of pupils in selective state schools join from the private sector.
In 50 grammar schools as many as a fifth of children were first educated in independent schools, while in 20 grammars the rate is at least a quarter.
The disclosure - in a report by Professor David Jesson, of York University - underlines the extent of the competition to get into England's 164 remaining grammar schools. A survey last month suggested half of parents send children to private tutors to pass the 11-plus entrance exam.
At the most sought-after schools, as many as 10 children compete for every place.
Experts warn the trend is being fuelled by middle-class parents who have been priced out of the independent sector.
Many are prepared to pay several hundred pounds for professional coaching to ensure children pass the 11-plus - rather than spend thousands every year on senior school fees.
"Private schools are not required to follow the national curriculum and are therefore free to coach for grammar school entry tests - one reason why parents choose these schools in the first place," said Prof Jesson. "The contrast with pupils in state primary schools could hardly be sharper. These pupils have, by law, to follow the national curriculum and any coaching would have to take place outside of school and at substantial additional expense."
Prof Jesson analysed data on pupils at 134 grammar schools.
He said those who arrived from state primary schools had better Sats results than those who took them in the private sector.
But privately-educated children performed better in the 11-plus, suggesting these tests were set as their priority.
The high attendance rate at grammar schools come despite the fact that only one-in-14 children are actually educated in fee-paying prep schools.
"It is surely very surprising to find that, with such competition for entry, able pupils from the state sector appear to be discriminated against in favour of less able pupils from the private sector," he said in the journal Research Intelligence.
By Graeme Paton, Last Updated: 7:12PM GMT 23 Jan 2009