John Mulholland, chairman of the Association for Quality Education (AQE), slammed proposals in a letter to school principals and governors.
There are currently two separate transfer tests in Northern Ireland, after the 11-plus exam run by the Department of Education was scrapped over a decade ago.
Proposals for a common test were put forward last year by negotiators representing the organisations that run the two exams; the Post-Primary Transfer Consortium (PPTC) and AQE.
The test would consist of two papers, with the best score of the two tests determining the outcome for each pupil.
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The PPTC – whose test is mostly used by Catholic maintained grammar schools – last week said there was “overwhelming support” from the 34 schools it represents for the creation of a single transfer test.
BBC News NI has also reported that a majority of grammar school principals have expressed support for the proposals.
However, Mr Mulholland wrote to grammar school principals and governors on behalf of the AQE board to voice his concerns about the plans.
While he said the board remains supportive of the concept of a joint test, he added: “It does not accept it should be adopted at any price.”
Mr Mullholland expressed concerns about the quality of the proposed common test.
He warned the common test would be a “halfway house”, which would be less reliable than the current AQE exams.
“The three-test format is key to the current assessment and the board have been convinced this approach offers the best opportunity to children, particularly to those from less advantaged backgrounds,” said Mr Mulholland.
“Advice received indicates the new arrangements could be more easily manipulated by middle class parents.”
He also voiced concerns that parents would have to pay much more for the single test.
Parents pay a £50 fee to enter their child for the AQE tests unless they are eligible for free school meals.
The PPTC does not charge a fee for entrance to its test.
“The current proposal takes the form of a compromise whereby entrants will be charged £20 with schools providing the balance, which could be in the region of £6,000-£7,000 and potentially considerably more,” Mr Mullholland wrote.