The introduction of universal free school meals for infants will mean staff cuts and the loss of thousands of pounds for disadvantaged pupils, headteachers fear.
They are worried that pupil premium funding - worth pound;1,300 per child - will not be allocated because it depends on parents registering their children as eligible for free school meals (FSM). From this term, all four- to seven-year-olds are entitled to free lunches regardless of their parents' income, so a major reason for registration has been removed.
Martin Henderson, headteacher of Westmorland Primary in Stockport, said: "We stand to lose money if parents do not apply for free school meals on entry to our reception class. There is knowledge that meals are universal now, so there is that confusion. The incentive has gone."
He said that 60 per cent of his pupils were eligible for FSM but some new parents had not registered this year because they knew their children would get lunches anyway.
"At the moment, we think we stand to lose about pound;15,000," Mr Henderson said. "That may not sound like much but it amounts to a member of staff. The value of that teaching assistant working with a child cannot be denied."
The government's national pupil premium champion Sir John Dunford said schools were "rightly concerned" about the issue.
The Department for Education (DfE) has suggested that schools ask all parents to register for school meals. But headteachers say this will mean more bureaucracy and point out that it can be difficult to get parents to fill in forms.
The NAHT headteachers' union wants central government to take responsibility for identifying children who qualify for the pupil premium.
Another concerned headteacher, Debra Bailey from Rushey Mead Primary School in Leicester, said: "If the children are having meals anyway, the parents will not apply for free school meals because they won't need it."
She added that it would be a "shame" for their pupil premium funding to be reduced, as the extra money - pound;180,700 last academic year - was spent on staff, subsidised school trips, pastoral support, links with parents and resources like tablet computers for children from disadvantaged homes.
"If the government wants all children to have meals at school, they are going to have to find a different way of identifying pupil-premium children," Ms Bailey said.
Russell Hobby, general secretary of the NAHT, said that he had received reports of schools in London with similar problems and described it as "the next big issue for the government to sort out".
"We think the system needs to move from local registration because the government knows who these people are," he said. "It should just tell schools and give them the money. Headteachers have better things to do than running around trying to find out who is on benefits when that data already exists."
Mr Hobby added that some families would not fill in forms whatever the incentive. "It might be because they don't understand them or they are suspicious of authority or they don't want to be tracked," he said.
Pupil premium champion Sir John said: "This is a widespread concern. But the lessons from the pilot in several local authorities were that it is possible to keep up the level of registration."
A DfE spokesperson said the way the pupil premium was calculated would not be affected by the introduction of universal FSM. It was still possible to identify pupil premium children through processes such as enrolment, they said.
The spokesperson added: "Many schools are asking all parents to register for school meals to make sure all eligible pupils are identified and get the funding they are entitled to.
"There continues to be a very strong incentive for parents to sign up - pound;1,300 per child means significant extra support for their child."
Chris Hassall, headteacher of Taylor Road Primary School in Leicester, fears that universal infant free school meals could mean a drop in funding for his school. However, he says that explaining the advantages of the extra funding to parents can minimise the problem.
"I tell parents about all the terrific benefits that their children can get if they get pupil premium," he says. "I think there are a lot of schools who haven't individualised the pupil premium yet and are just lumping it all together. But the money should go directly to the child that it is meant for.
"I tell parents that their child won't pay for any educational trips, they will get free school uniform, sports kit and breakfast. I explain that they will be entitled to one-to-one tuition irrespective of whether they are underachieving. That is how I have sold it."