Figures from exam boards revealing that grammar schools overtook private schools in GCSE performance this year, have provoked a row as the independent sector argued that the comparison was misleading.
State grammars were ahead of their private counterparts at A grade, with 51.5 per cent of their entries achieving this mark compared with 50.8 per cent among independents.
The corresponding figures for C grades and above were 94.4 per cent for grammars against 90.7 per cent for independents.
Private school results at both grades have fallen back slightly since 2005.
The Independent Schools Council said the falls were a result of many high performing schools dropping GCSEs for the International GCSE, which does not feature in the figures.
However, Greg Watson, chief executive of the OCR board, which offers the IGCSE, said the numbers taking them were too small to explain the change.