Ofsted chief Sir Michael Wilshaw says a flagship policy to target extra funding at poor pupils in England is having little impact on many schools.
A snapshot survey of 117 schools in poor areas suggests the extra £600 per poor pupil a year made little or no difference to support for them.
Sir Michael said it was a "real worry" if cash was being diverted to "tarmacking playgrounds".
Schools minister David Laws said schools would be held accountable.
Under the scheme, schools in disadvantaged areas in England are allotted £600 per head to help give extra support to poorer children. Those from disadvantaged backgrounds often do not do as well as their more advantaged peers and the funds are aimed at helping to bridge the gap.
The National Union of Teachers said the funding was being used in many schools to plug holes in their budgets.
The Ofsted report, which surveyed and inspected nearly 300 schools in total, found that half the schools thought the pupil premium was having a positive impact on raising achievement, but it said few could provide evidence to back this up.
Speaking about the results of the snapshot survey on BBC Radio 4's Today programme, Sir Michael said: "We found that over 50% said that it was having either little or no impact on the way they organise and manage their schools in relation to the use of money on poor children.
"We find that surprising - this is a large chunk of public money."
Russell Hobby, general secretary of the National Association of Head Teachers, said the government had introduced the pupil premium at the same time as cutting school budgets elsewhere, so overall the amount of money in the front line in schools was exactly the same after the pupil premium.
"Actually what you see in Ofsted's data is that schools have invested in extra teachers and extra teaching assistants, by and large, so that they can give extra help for these pupils.
"So I think (support) is going to the disadvantaged pupils. What I worry about is, is that the best way to help them?"
Christine Blower, general secretary of the National Union of Teachers said the premium's "low rate and uneven distribution" meant it was not sufficient to protect schools against cuts to their funding.
"Inflation will soon erode the value of an already-frozen schools budget. As a result of this funding crisis schools are being placed in the untenable position of having to use the pupil premium to plug gaps in existing provision and not for entirely new initiatives," she said.
The pupil premium is a key coalition policy, initiated by the Lib Dems.
Mr Laws said the government would not wish to micromanage schools.
But he added: "Critically, while we're giving those schools freedom to use the money as they think best, we are also putting in place an accountability mechanism which will ensure that they use the money in the right way."
He said this meant that during visits to schools, Ofsted inspectors would "look at whether the schools are closing the gap between advantaged and disadvantaged pupils."
Then inspectors would "look at the ways they are using the money" and be "critical" where this was not effective.
Martin Johnson, deputy general secretary of the Association of Teachers and Lecturers said: "Ofsted is supposed to make independent judgements. Its pupil premium report shows that it is now simply a ministerial mouthpiece, giving more credence to poor quality research and political spin.
"Schools are suffering real cuts to funding, and rumours are circulating of a big hit in the Chancellor's autumn statement, so it would be perverse of schools not to use the pupil premium to plug gaps in their funding."
The report comes as other Liberal Democrat policies come under scrutiny, with party leader Nick Clegg and Business Secretary Vince Cable apologising for breaking their party's pledge to oppose increasing student tuition fees.
The shadow schools minister Kevin Brennan said: "A year and a half after being introduced, this report shows that the pupil premium is not working in the way it was intended, because it fails to offset the cuts the Tory-led government has made to the schools budget."