The country's biggest fully selective local authority has admitted that the 11-plus entrance exam used to select grammar school pupils favours wealthier children.
Buckinghamshire County Council made the candid disclosure in a letter responding to a school that was considering using the test to select students from different ability groups as part of a fairer admissions policy.
The council's comments came just weeks after the row over selective education erupted again after a decision by Kent County Council to allow grammars to open satellite schools in order to cater for demand.
Buckinghamshire council's admission was made in response to Highcrest Academy in High Wycombe, which will become the first fully comprehensive secondary in the county in September, after its recent conversion to academy status. As an academy, the school can set its own admissions policy and had intended to use a fair banding system using the results from the 11-plus, which every primary pupil in the county is expected to sit.
But in its response to Highcrest's plans, Buckinghamshire town hall warned the academy against using the test because "affluence" played a role in the entrance exam.
In its letter, the council wrote: "In the application of any test (11- plus, CATs or SATs), we know that affluence is a factor, probably a stronger factor than ethnicity."
Buckinghamshire is one of the few remaining fully selective counties in England, and its admission that the test it uses to do this favours children from richer backgrounds was described by Highcrest's head, Sheena Moynihan, as "unbelievable".
"They acknowledge that affluence is a factor and, by implication, that ethnicity is a factor as well. And this is the test they use," Ms Moynihan said. "This is what they said in their objection letter to us using the 11-plus exam for our admissions policy.
"That sentence decided it for us that we would definitely not use the 11- plus as part of our admissions," she added.
Critics of the entrance exam often point to the instrumental role that private coaching can play in helping pupils to pass the 11-plus, complaining that it is generally more well-off families who can afford to pay for the extra tuition for their children.
In addition, the verbal reasoning part of the 11-plus is crucial, according to Ms Moynihan, who argued that this disadvantages children with English as an additional language and those with dyslexia.
A spokeswoman for the anti-selection pressure group Comprehensive Future, which campaigns for the abolition of the 11-plus, said she was "surprised" by Buckinghamshire's frank admission, but added that what the council had said backed the group's argument against selection.
"The comment supports what we have been saying for some time, that the 11- plus is not a fair test," said Margaret Tulloch, Comprehensive Future's secretary. "It is for this reason that we think the 11-plus should be abandoned, and if selection was abandoned then admissions would be a lot easier for everyone."
However, supporters of the grammar school system described as an "excuse for failure" the belief that affluence or ethnicity was a factor when sitting the 11-plus.
The National Grammar Schools Association, a lobby group that campaigns for the expansion of the grammar school system, said the 11-plus was based on academic ability, not financial status. "This idea that how much money you have will dictate how well you do is completely false and, I believe, an excuse for failure," said Nick Seaton, an executive member of the NGSA.
In a statement, the council said that it had merely been pointing out that children from deprived backgrounds were more likely to be withdrawn from the test by their parents.
"Buckinghamshire County Council, as the local education authority, submitted a formal response to the consultation held by Highcrest Academy on their proposed changes to admission arrangements for 2013," a council spokesperson said. "In our response, a reference to affluence was made purely in the context of factors which may influence whether parents wish their children to sit the 11-plus tests."
13 - Number of grammar schools in Buckinghamshire
21 - Number of secondary modern or "upper schools" in the county
30% - The proportion of children who gain a place at a grammar school
121 - The pass mark for the 11-plus
142 - The top mark obtainable in the 11-plus.
Original headline: Wealth matters when it comes to the 11-plus, says selective council