Sir Robert Balchin, chairman of the Grant-Maintained Schools Foundation, said he had received letters from heads who are frightened for their school and their personal futures if Labour gets into power. He said: "When many schools decided to opt out they believed John Patten's promise that grant-maintained schools would eventually be in the majority. Now they are beginning to feel very vulnerable."
Opted-out heads have also voiced concerns to Pauline Latham, chairman of the Grant-Maintained Schools Advisory Committee, about their future relationships with local authorities if they lose their special status. Labour's proposals would see GM schools become foundation schools and receive the same funding as community (former county) schools and aided (denominational) schools.
The fear within the sector is that local education authorities, which would have a greater role in inspection and standards under a Labour Government, would make life difficult for former opted-out schools.
A GM governor said: "Tony Blair may talk about New Labour and Christian socialism but many councils, especially in the North, are entrenched Old Labour with less charitable views and we fear they will act vindictively."
Opted-out schools fear they may get a raw deal on capital funding from LEAs, who believe they have done well under the present system. And schools who have been paid twice for services they previously received from the LEAs are among those likely to feel most vulnerable.
Ian Plimmer, head of Bingley Grammar School, the first in Bradford to break the link with its LEA, is concerned about cuts to funding.
"My main worry is that staffing would be hit if the 8 per cent of the local authority budget we get to cover central costs were withdrawn and we had to use the council's services instead," he said.
"Having the freedom of managing the school with the senior staff and governors has made an enormous difference to us but there are councillors who still think schools should be controlled by the local authority. It would be a very retrograde step if they tried to claw power back."
For Eamonn Harris, head of Queen Elizabeth Boys in Barnet, the biggest fear was of the unknown. "There is no clarity about the way ahead and that's what makes me feel vulnerable. The Labour party doesn't have anyone with a track record and how people who have planned in opposition will behave in Government remains to be seen."
Brigid Beattie, head of Burntwood girls' GM school in Tooting, Wandsworth, was concerned Labour might try to end selection. "Some GM schools have moved towards partial selection and that would be threatened. I'm sad about that because it would mean less choice for girls." But she added: "It would be very difficult for Labour to restore local authority power because GM schools are very popular with parents."
Richard Townsend, head of Dunraven in the London borough of Lambeth, said any school - GM or LEA - should only feel vulnerable if it were not doing its best for its pupils.
While Neil Slater, head of Dover School for Boys, Kent, said: "We do have fears - fears of being used as a party political football - but I think all schools fear that, not just GM schools."