Private schools which are registered as charities could be forced to show they benefit the public, under government plans.
Private schools say proving their public worth is easy
About 80% of independent schools are listed as charities, gaining tax benefits.
Under the draft Charities Bill just published, those seeking charitable status in England and Wales will have to show they benefit "the public at large".
The plans could put pressure on fee-paying schools to offer more places to
children from less well-off families.
Independent Schools say they are confident they can be seen as benefiting the public.
Under the Bill, the Charity Commission would be expected to devise a test of "public benefit" which all charities would be expected to pass.
Fiona MacTaggart, the Home Office Minister with responsibility for charities, said the point of the Bill was to make sure charities' abilities to do good are protected.
"We must protect the charity brand so that people are confident about giving money to charities and know what is and isn't a charity," she said.
Ms Mactaggart rejected claims that fee-charging institutions, such as independent schools or private hospitals, do not serve the public interest
because they are not available to all.
"I do not believe that charging means you do not automatically provide a
public benefit," she said.
Earlier in May, public schools were warned they could lose their charitable status unless they did more to help their local communities.
Some do already open up their facilities to neighbouring state schools or the community in general.
The government is trying to encourage co-operation and sharing of resources and even lessons between state and private schools.
Dulwich College has announced plans to open its own state comprehensive in east London.
Jonathan Shephard, general secretary of the Independent Schools Council, told BBC Radio 4's Today programme that public schools could demonstrate they were doing enough to benefit the public at large.
"We educate half a million children at no cost to the taxpayer. If the taxpayer had to pay for those children, it would add £2bn to the tax bill," he said.
"We also share our facilities, our teaching materials and our staff," he added.
There is also provision in the Bill to overturn a historical quirk which meant that top public schools Eton and Winchester did not have to register as charities.
They have been listed as "exempt charities" and until now have not had to supply accounts to the Charities Commission, although they had to make accounts available.
The Bill will remove the schools from the exempt list and put them on the same basis as charitable schools.
A spokesman for the Independent Schools Council said this was a technical change for which the schools would have prepared.
The draft bill also seeks to crack down on so-called charity muggers - that is canvassers who seek donations on the street.
At the moment only charities which are collecting cash need a licence - those collecting direct debit pledges do not.
Under the bill's proposals, licences could be withheld if the local authority thought there would be too many such workers on the street at any one time, causing a nuisance to the public.