Grant-maintained schools are the most expensive places to take three A-levels in the state sector, according to the Government's own figures.
New statistics from the Department for Education and Employment reveal the cost to the tax-payer of students taking three A-levels is greater in GM schools than in local authority-run schools, further education or sixth-form colleges.
The costings will embarrass the Government, giving opponents the chance to accuse ministers of pouring additional cash into their favoured institutions.
It will anger critics all the more this week since the selection White Paper gives GM schools powers to open new sixth forms without formal approval.
The new figures have been issued as part of a DFEE study of comparative costs of education and training for 16 to 19-year-olds in school sixth forms, colleges and work-based training. They show that GM schools receive Pounds 6,645 per student taking three A-levels - Pounds 260 more than LEA schools.
The statistics have been seized on by Labour as evidence of funding partiality. Education and training spokesman Stephen Byers called it a "political own goal".
He said: "The Government has constantly proclaimed GM schools as being both cost effective and offering high standards. These figures would appear to show that in relation to other sixth-form providers they are doing neither."
The latest figures on comparative costs are a revised version of a set issued by the DFEE last December in which GM schools did better. The earlier set put the cost of three A-levels in a GM school at Pounds 6,115 - Pounds 530 less than the new version suggests.
The December statistics placed sixth-form colleges at the top of the cost league, with GM schools said to be only marginally more expensive than LEA schools or FE colleges.
The revised figures will give some comfort to colleges, which were infuriated by the earlier version. They insisted they were better value than schools, which they claim cross-subsidise sixth forms using cash intended for younger pupils.
The DFEE's basis for its calculations was widely questioned.
The cost comparison was launched following the announcement in the 1995 Competitiveness White Paper that the Government would "investigate whether there is a case for encouraging a more consistent approach to funding methodologies across the sectors". Many suspected a hidden agenda to "prove" a level playing field in terms of costs in order to pave the way for post-16 vouchers, but the DFEE has now ruled these out.