Last week the Labour conference agreed to abolish the Assisted Places Scheme. The Association of Heads of Grant-Maintained Schools says that such proposals will release money into the education system.
"The opportunity exists, therefore, to bring the level of delegated funding of all schools up to those of GM schools, not to cut it back. Schools certainly should have greater control over their funding," it said.
GM heads see little benefit in becoming "foundation" schools - one of three types proposed in Labour's policy document Diversity and Excellence which was approved by the conference.
Under a Labour government schools would choose their status in consultation with parents. They could be either: community schools - based on existing secondary schools with one extra parent-governor; aided - based on voluntary-aided schools which receive 85 per cent of capital budget from the local education authority; or foundation - the governing body would employ the staff, but each body would have to include two education authority representatives.
All three would have a maximum of 10 per cent of their budget held back by the education authority and their admissions policy would be based on parental preference agreed in consultation with the authority.
In a draft response to Diversity and Excellence, the AHGMS said: "A target of 90 per cent delegation to foundation schools is a clear 10 per cent cut to GM budgets. No justification is given for this yet the implications for redundancies are starkly clear. Indeed, such a reduction is in complete contradiction to the express commitment to increase delegation."
The heads complained that local authorities already retained an average of 17 per cent of money for all schools, including GM ones, to pay for responsibilities such as special education. Some authorities, such as the London borough of Lambeth, top-sliced as much as 25 per cent.
And they said: "If the target is for only 90 per cent of what remains to be delegated, then LEAs will still be holding on to more than Pounds 25 in each Pounds 100."
The GM heads called for a national funding formula for schools and criticised Labour's moves to put education authority representatives on the governing bodies of foundation schools as "unnecessary interference".
"The overriding principle must be that governors should be acting in the best interests of the school and not find themselves in the position of having split loyalties."
They said GM schools should be able to continue using the admissions procedures they had developed and added: "In areas where testing takes place, schools should be allowed to administer their own tests."
Labour would re-establish the monopoly of local authorities and remove the challenge provided by GM schools, said the AHGMS. It added: "The effect of Labour's proposals in answer to the question 'Who controls schools?' is that the LEAs will.
"If the underlying principle was genuinely 'LEAs do not control, schools do', there would be no valid reasons to change the existing structures of GM schools.
"There is no better means of achieving the principle 'LEAs do not control schools, schools do' than preserving the existing arrangements for GM schools. "