Under new laws, charitable organisations are expected to prove they exist for the wider "public benefit". But schools that charge high fees and are not open to poorer children will fail this test and face losing the tax breaks that come from being registered as charities.

The Charity Commission, which will police the new laws, suggested independent schools give lessons to state school pupils or lend teachers to comprehensives.

The Commission said fee-charging independent schools will be among the first to be assessed and could receive random visits from officials.

In new guidance the watchdog said: "At the extreme, charities should not be seen as 'exclusive clubs' that only a few can join, since the 'public' benefit from that is very limited."

The Commission said charities can still charge "reasonable and necessary" fees.

It has been estimated that private schools receive £100 million a year in tax breaks through their charitable status.

Latest figures showed more than 500,000 children attend UK private schools, with average fees in 2007 rising to £11,100 per year.

The guidance followed warnings from a leading independent headmaster that private schools have deepened class divisions by depriving state comprehensives of their brightest pupils.

Anthony Seldon, master of Wellington College, accused Labour and the Conservatives of perpetuating the "apartheid" between state and private schools.