Government plans for a national funding formula for grant-maintained schools have been condemned by the sector's advisory body.
Heads of opted-out schools have long pressed ministers to standardise funding, but they believe it would be wrong to limit a national formula to the 1, 000 GM schools.
Cecil Knight, chairman of the GM Standing Advisory Committee, said his schools could lose out to local authority schools. He said a national formula under which children attract equal shares of funding would be fairer and easier for parents to understand.
The Government has shied away from a national funding formula as it would produce winners and losers. Even now, it says, such a system would not be introduced before the general election.
However, last September John Major said that national funding was the way forward for the opted-out sector in the long term and last week schools minister Robin Squire launched a discussion paper on a formula solely for the GM sector. It contained an element of payment-by-results with ministers arguing that this was a powerful lever to improving achievement.
Civil servants have warned that some schools could lose up to 22 per cent of current budgets while others could gain as much as 17 per cent. Differentials could be quite large within individual local authorities.
Mr Knight, head of Small Heath school in Birmingham, said: "A national funding formula only for GM schools is unacceptable. You would have GM schools which would be at a disadvantage against other LEA schools. A national formula must be for the nation's schools, not just GM schools. We have never set out to put ourselves at advantage - we want no more or no less than is our fair share. "
Ministers believe a funding formula for the GM sector would cut the last ties opted-out schools have with their former local authorities. They claim also that such a formula is the natural next step from the common funding formula. This is already operating in 23 local authorities where at least 30 per cent of secondary pupils attend GM schools.
The principle of payment-by-results has already been established in further education. However, Mr Knight said that such payments should be calculated only on the basis of the value added to a child's achievement by the school. Work on value added is still in its infancy.
Ministers have given schools and interested bodies until September 23 to comment on the discussion paper.