Schools are being forced to use the Government's flagship pupil premium to plug cuts to their budgets, headteachers are warning.
More than four-fifths of heads say the money has either equalled or not made up for financial losses elsewhere, according to a survey carried out by the National Association of Head Teachers (NAHT) for the Press Association.
Many also remain unconvinced that the premium will be beneficial to their poorest pupils, with just over a third saying they do not think it will make a difference to a student's achievement.
The pupil premium, a key initiative for the coalition Government, is extra funding attached to disadvantaged children, following them as they move schools.
It is given to pupils who are eligible for free school meals (FSM) - a measure of poverty - with the aim of closing the achievement gap between richer and poorer youngsters.
According to a written ministerial statement made last week, the money is in addition to the underlying schools budget, and stands at £600 per pupil for 2012/13, up from £488 last year.
The NAHT's survey, which questioned more than 2,000 heads, found that a third (32.4%) said the premium had equalled losses elsewhere in their budget, while more than half (53.3%) said it had not made up for losses elsewhere.
Just 14.3% of those questioned said the premium had exceeded losses elsewhere in their budget.
As the funding follows a pupil through their schooling, schools with large numbers of FSM youngsters will receive more money, while others get less.
NAHT general secretary Russell Hobby said: "The pupil premium seems to us to be the right thing to do as a concept, it's the right idea. But in today's climate it is simply redistributing funds in the system, not adding more."
He said it could be argued that it is extra money for the poorest pupils, but added "some children will get less as a result".
"You could argue that's fine and they need it less," Hobby said.
The survey findings, which come as the NAHT prepares to meet for its annual conference in Harrogate this weekend, show that nearly half of those questioned (48.9%) did not think the pupil premium was enough money, a third (33.6%) were not sure, and 17.6% said it was enough.
And just over a quarter (26.8%) said it would make a difference to a pupil's achievement.
More than a third (35.4%) said it would make no difference, while a similar proportion (37.9%) were unsure.
Heads were also asked to say what they thought the premium could pay for in reality.
Of the more than 1,000 answers given, the most popular was extra teaching assistants, teachers, support staff and teaching time, followed closely by additional support such as one-to-one or small group tuition.
Schools were also using the money to pay for resources such as books and computers, while others were spending it on school trips an extra-curricular activities for their poorest pupils, with a couple saying they used it to pay for uniform or lunches.
But around a fifth of the answers given were that the premium paid for very little, either in a particular school or more generally.
Hobby said: "By and large the frontline budget is stable, but that's with the pupil premium included in it.
"Where the education budget is disappearing is at local authority level, where they are cutting 30%-40% over the next few years."
Services such as music tuition and support for special educational needs are being cut, with schools now buying them in themselves, he said.
"Simply, as it stands, the pupil premium is not enough to make a significant difference. With around £400 this year it's not going to go very far for some of these problems."
Hobby said the focus on one-to-one tuition was "interesting" as it was "really effective but really expensive".
From this September, schools will be required to publish information on how they are using the premium to benefit poorer children, and Hobby said "people may be worried about being accountable for the money - that's why they're going for one-to- one tuition".
A Department for Education spokeswoman said: "The pupil premium has been designed specifically to help schools to improve the attainment of disadvantaged pupils.
"We have protected the overall schools budget in cash terms so there should be no need for schools to use the premium for anything other than this purpose.
"From September, schools will have to publish information showing how they have spent the premium and what the impact of that spend was. Schools tell us they have used this extra money to pay for catch-up lessons, one-to-one tuition or after-school clubs."
But shadow education secretary Stephen Twigg argued: "With more than half of headteachers saying it doesn't even make up for the cuts they have had in their budget, it is clear that deprived pupils are suffering under this Tory-led Government.
"Most heads are unconvinced the pupil premium will make a difference to achievement. The Government needs to introduce far clearer accountability measures to ensure resources are targeted and effective at helping those pupils who need it most."