Private schools should be stripped of tax breaks unless they sponsor city academies or take other significant steps to help youngsters from deprived areas, a Government adviser has said.
Alan Milburn, the ex-Labour cabinet minister who is now the independent reviewer on social mobility, said fee-paying schools were doing too little to justify their charitable status.
The independent sector won a long-running court battle late last year to force changes to stricter new Charity Commission guidance on what its institutions had to do to retain the status, which saves large sums in tax.
A tribunal said there should be more freedom for the trustees of a school to decide how it met an obligation to provide a wider public benefit - beyond simply bursaries and other support for disadvantaged youngsters.
Mr Milburn, who helped push through the tougher regulations in 2006, complained that schools were able to get away with doing little to justify the special status.
"Frankly the tax break shouldn't be given just because on occasions the private school opens its playing field to the state school. It's got to go beyond that," he told The Times.
"I think the Charity Commission got it wrong in the way they interpreted the public benefit test. I would make a condition of charitable status for private schools that, for example, they sponsor a city academy. That's putting something back.
"There might be other things they could do - take more kids from disadvantaged areas, for example. We've got to move for a something-for-something deal."
Prime Minister David Cameron called for more private schools to sponsor state-sector academies in his party conference speech last year - saying he wanted to tear down the "apartheid between our private and state schools".
But progress has been slow despite him summoning heads to a summit on the issue.