Plans announced this week to offer places at some of the country's leading private schools on a "needs blind" basis will fail without government money, according to the educational philanthropist Sir Peter Lampl.
The founder of the Sutton Trust, which aims to increase opportunities for children from disadvantaged backgrounds, praised an announcement by St Paul's school, London, to offer its places purely on academic merit, but said the amount of money needed for it to succeed was "astronomic".
Martin Stephen, the high master of St Paul's, said the school was embarking on a massive fundraising drive to be able to offer all of its places to pupils regardless of their parents' income within 25 years.
It has been estimated that the school will need to raise an endowment fund of at least pound;200 million to cover the day fees of pound;14,000.
Mr Stephen said: "I think the figure has been overestimated. It will be high but I'm far from being daunted. These things only happen if people want them passionately to happen and we do.
"The point is that intelligence is one of the few natural resources we have left in this country and it is wrong to stop children from all backgrounds from being given the best chance to develop their academic abilities."
But while calling Mr Stephen's announcement a "great aspiration", Sir Peter said it would not be possible to privately raise the kind of money needed for open access to independent schools.
He said: "The only way to do it properly is with government funding. The vast majority of universities don't have endowments of over pound;100m, let alone secondary schools," he said. The Sutton Trust, in partnership with the Girls' Day School Trust, set up a scheme in 2000 to offer places at Belvedere girls' school, Liverpool, on academic merit. A report last year by Professor Alan Smithers, of Buckingham University, found the project had succeeded, with the school improving its social mix while achieving its best ever GCSE results.
Along with George Walden, ex-Conservative higher education minister, Sir Peter is calling for the Government to fund an extension of the Belvedere experiment to 12 schools to see if it can work on a larger scale. He would then like to see the initiative rolled out to 100 schools across the country.
He said: "At Belvedere we pay around pound;4,400 on average towards each student's fees of about pound;8,000. That is less than the cost of a state school place. Of course, selection is a big issue for the Government but it is happening anyway. At the moment it's just on ability to pay, but it should be on children's abilities."
St Paul's announcement comes as the Charities Bill, requiring independent schools to demonstrate their "public benefit" in order to maintain their charitable status, returns to Parliament.
Alan Johnson, the Education Secretary, has in several speeches, the latest at Labour's Manchester conference, called for independent schools to do more to justify their status, which saves them around pound;100m a year in tax breaks.
But Mr Stephen said St Paul's decision was not linked to the Charities Bill, saying the legislation had "never caused me the slightest flicker of concern".