They suggest that England's 166 grammar schools contain a far lower proportion of poor children than non-selective state schools.

In Kent, which retains a selective education system, nearly 13% of children who attend secondary schools receive free school meals.

However, just 3.4% of those who attend grammar schools in the county are on free school meals.

The findings have been siezed upon by opponents of selective education.

'Simplistic'

Margaret Tulloch of the Campaign for State Education said the figures demonstrated that selection resulted in social exclusion.

But John Edwards, headmaster of Harvey Grammar School in Folkestone, Kent, said such an analysis was too simplistic.

"While it is true that children from middle-class backgrounds are more likely to succeed, we do have a number of children in this school who come from very difficult backgrounds," he said.

Regulations which passed into law last year allow for the abolition of selective education in a particular area if a majority of parents affected by such a move vote for it in a ballot.

Ballots can only be triggered by a petition signed by 20% of those eligible to vote.